greater than the sides of the triangle which includes it, in any Take a straight line D E termi- Scholium 2.-Respecting this proposition also, Dr. Thomson very properly remarks, that it "is never referred to by Euclid [in his after writings], except in the eighth proposition of the third Book; and that proposition may be proved without it." The Prop. VIII. Book III. is, in fact, proved without it, in Cassell's Euclid. Thus, it would appear that one of the links of the great Euclidean chain of reasoning is unnecessary. attempts to shorten that chain are no doubt praiseworthy; for art is long, and life is short. Nevertheless, the proposition contains a geometrical truth, useful in many cases of which Euclid never dreamed; and, therefore, we ought to retain this extra link. EXERCISE TO PROPOSITION XXI. All lines σ A, C B and CE is less than the sum of the three sides A B; BE and AE; but greater than half their sum. By the preceding proposition, a C and CB together are less than AR and E B together; BC and E c together are less than A Band AE together: and Ac and CE together are less than AB and BE together. Therefore, by axiom IV. twice Ac, twice BC, and twice c E together, are less than twice A E, twice в E and twice AB together; wherefore, a C, B C and CE together are less than A E, B E and A B together. Again, by Prop. XX., A c and cв together are greater than AB; EC and CE together are greater than BE; and A C and CE together are greater than AB. Therefore, by Axiom IV., twice a C, twice BC, and twice CE together, are greater than AB, BE and AE together. Wherefore, A C, BC, and CE together, are greater than the half of AB, of B E and of A E together. Therefore if from any point within a triangle, &c. Q. E. D. Scholium.-In this demonstration, a very obvious axiom is taken for granted, viz. that the halves of unequals are unequal; or, as it may be more explicitly expressed, if the double of one series of magnitudes taken together, be equal to the double of another series of magnitudes taken together, the sum of the former series is equal to the sum of the latter series, &c. PROPOSITION XXII.—PROBLEM. To make a triangle of which the sides shall be equal to three given straight lines, but any two whatever of them must be greater than the third (I. 20). In fig. 22, let A, B, and c be the given straight lines, of which any two whatever: are greater than the third, viz., a and B greater than o; A and a greater than B; and B and c greater than a. It is required to make a triangle of which the sides shall be equal to A, B, and c, each to each. Because the point F is the centre of the circle DK L, the straight line FD is equal (Def. 15) to the straight line FK. But FD is equal (Const.) to the straight line a. Therefore FX is equal (Az. 1) to A. Again, because a is the centre of the circle LEH, the straight line GH is equal (Def. 15) to the straight line GK. But GH is equal to c. Therefore also GK is equal to c. And FG is equal (Const.) to B. Therefore the three straight lines KF, FG, and G K are equal to the three straight lines A, B, and c. Wherefore the triangle KF has been made, having its three sides KF, FG, and GI, equal to the three given straight lines A, B, and c. Q. E. F. Scholium 1.-This is the general proposition of which Prop. I. is but a particular case. It is evident that upon the other side of the base FG, another triangle might be constructed, having its three sides equal to the three given straight lines. In the demonstration, it is assumed that the two circles will intersect each other. To prove this, it is sufficient to observe that the sum of the radii of the two circles is, by hypothesis, greater than the distance between their centres. Scholium 2.-Dr. Simson remarks on this proposition, that some authors blame Euclid, because he does not demonstrate that the two circles made use of in the construction of this problem, must cut one another: but this is very plain from the determination he has given, viz. that any two of the straight lines D F, FG, GH, must be greater than the third. For who is so dull, though only beginning to learn the elements, as not to perceive that the circle described from the centre F, fig. 22, at the distance FD, must meet F E betwixt F and ¤, because FD is less than FH; and that for the like reason, the circle debetwixt D and G; and that these circles must meet one another, cribed from the centre G, at the distance GH, must meet DG ing, however, notwithstanding his predecessor's remarks just because F D and G H are together greater than FG?" Forseecited, that some learners might be so dull as not to perceive what seems so clear to a geometer, Dr. Thomson has very properly inserted the following paragraph in his construction, after the words "another circle HLK; and to this insertion all beginners would do well to take heed: DK L. ference of the circle DKL will cut F H between F and H; and "Now, because (hyp.) FH is greater than FD, the circumtherefore, the circle K H L cannot be wholly within the circle the circle DKL cannot lie wholly within the circle K H L. In like manner, because (hyp.) D G is greater than GH, Neither can the circles lie wholly without each other, since (hyp.) DF and GH are together greater than FG. The circles must, therefore, intersect each other; let them intersect in the point L." EXERCISE I. TO PROP. XXII. To make a triangle equal to a given triangle. straight lines of which any two are greater than the third, Consider the three sides of the given triangle, as three given Prop. XX., and by the preceding proposition, construct a triangle of which the three sides shall be equal to these three given straight lines. By Prop. VIII. it is plain that the angle contained by any two sides of the one triangle is equal to the angle contained by the two sides equal to them of the other and by Prop. IV. that the two triangles are equal to each other in all respects. Wherefore, a triangle has been constructed equal to the given triangle. Q. E. F. * * This exercise was solved by C. L. HADFIELD, Bolton-le-Moons ; Q. PRINGLE, Glasgow; T. BOCOCK, Great Warley; J H, EASTWOOD, Middleton; and E. J. BREMNER, Carlisle. EXERCISE II. TO PROP. XXII. To make a rectilineal figure equal to a given rectilineal figure. Divide the given rectilineal figure into triangles, by drawing straight lines from one of its angular points to every other angular point in the figure, except the two angular points adjacent to the assumed one. Then, the given rectilinear figure will be divided into as many triangles as the figure has sides, wanting two. Now, by the preceding exercise construct, at any assumed point, a series of triangles contiguous to each other, and such that each of them shall be equal in succession to the triangles into which the given rectilineal figure has been divided. Then, it is plain, by Axiom II. that the figure constructed of this series of triangles, shall be equal to the given rectilineal figure in all respects, viz. in sides, angles, and area. Wherefore, a rectilineal figure has been constructed equal to the given rectilineal figure. Q. E. F.* PROPOSITION XXIII.-PROBLEM. ANSWERS TO CORRESPONDENTS. J. T. E. (Nottingham): We thank the correspondent for his suggestions. There is much want of a good Italian dictionary, and the subject will receive due consideration. With respect to the use of the Italian defintte that it is impossible to prepare illustrative exercises where some points of article, our intelligent correspondent will have the goodness to consider the grammar, however few, should not be anticipated in order to relieve them from dulness. To explain all these points as they occur would overin the course of the lessons, but it is impossible to lay down a short rule on load the grammar with notes. Some general hints have been thrown out the subject, which will answer our correspondent's question as to the use of the article, there being too many points, delicacies and exceptions, all of which will be fully (practically as well as philosophically), explained in special previous exercises, and test and correct them by the progressive rules as they lessons. A careful pupil, like our correspondent, will not fail to recur to are stated. With regard to the special sentence mentioned, it may be briefly stated that the article la ought to be placed before temperanza, though an abstract noun; because temperance is here expressly stated to be an indivi dual possession-il tesoro del savio. At a given point in a given straight line, to make a rectilineal dent must have remarked that this is not the case in the Italian auxiliary angle equal to a given rectilineal angle. Fig. 23. S. T. (Chester): Personal pronouns are indispensable before English verbs, for want of inflections, e. g. have is the identical word in the first persou singular, and in the first, second, and third persons plural. Our corresponavere, to have, and on this account personal pronouns before Italian verbs are, generally speaking, not indispensable, though used where stress, contrast, distinctness, &c. demand. With respect to the use of the article quently used than omitted in such cases, and the possessive pronouns often are not considered (as in English) sufficient to determine the noun. The use and the omission of the article before possessive pronouns will be explained hereafter. It was stated in a note that the colloquial exercises alluded to by our correspondent are anticipatory ones, and it is a useful exercise to the student to apply his understanding to find out grammatical rules for himself. In fig. 23, let A B be the given straight line, a the given point in before possessive pronouns, it is a peculiarity of Italian that it is more freit, and DCE the given rectilineal angle. It is required to make an angle at the given point a in the given straight line A B, that shall be equal to the given rectilineal angle D CE. D C E F B G In CD and CE, take any points D and E, and join D E. and join D E. Upon the straight line AB make (I. 22) the triangle AFG, the sides of which shall be equal to the three straight lines CD, DE, and E C, that is, A F equal to CD, AG to CE, and FG to DE. The angle FAG is equal to the angle D C E. Because the two sides FA and AG are equal to the two sides DC and CE, each to each, and the base FG to the base DE. Therefore the angle FAG is equal (I. 8) to the angle DCE. Wherefore at the given point A, in the given straight line a B, the angle FAG is made equal to the given rectilineal angle DCE. Q. E. F. Scholium.-It is evident that upon the other side of the straight line A B, another angle might be made equal to the given angle D CE; and that thus an angle might be doubled. EXERCISE TO PROPOSITION XXIII. At a given point in a given straight line, to make an angle equal to the supplement of a given angle; also, to make an angle equal to the complement of a given angle. First Produce one of the legs of the given angle, and by the preceding proposition, make an angle equal to the angle contained by the other leg and the part of the line produced; but this angle is the supplement of the given angle. Wherefore, an angle has been made equal to the supplement of the given angle. J. S. S. We are glad that he is about to purchase all the back numbers of the P. E., and we wish that thousands would do the same; we are certain that they would find their advantage in it. They may be had through any bookseller at the regular published price; not otherwise.-A MASTER KEY (Stoke-Newington) has made some very good poetical lines, but we wish that he could spell and write better. J. W. MOXLEY (Hoxne), must learn to spell and write English, before ke can give advice about "disease and essences."-A CORNISH SUBSCRIBER; Brande's Chemistry.-ADIANTUM (Basingstoke): The Ferns are principally distinguished by the shape and size of their fronds, and the position of the fructification in them. A strong magnifying glass is necessary to distinguish many varieties. The specimen sent is one of the Asplenia or Spleen-wort. -A LEARNER (Hackney): No general rule for aspirating the letter h can be given.-J. SMITH (Theobald's Road): See P. E. vol. i. p. 221, cól. 1. The Lessons in English are finished. UN COMMIS (Manchester): We can't give you the information you require, as we have not been in France since the reign of Louis XVIII.-T. BRIM (Perthshire): Snowball's Trigonometry.-J. C. FIELDEN (Blackburn) Boy, who could in a few minutes multiply mentally a row of 6 or 7 figures wishes to know something about Zerah Colburn, the American Calculating by a row of the same number of figures; as he says he can do the same in 1 minute and 15 seconds correctly, the sum to be set by any person. W. A. G. (N. Folgate): The remaining sections are omitted in the P. E., but will be found in Cassell's "Lessons in French," Part II., so far reprinted from the P. E.—AN ESQUIMAUX (London) should drink cod-liver oil; he will then grow much less than twelve inches, the next four years; his latitude will exceed his longitude.-COLOMBUS (Manchester): The Map of England and Wales is contained in No. 66 of the P. E. The cheapest drawing instruments are to be had at the" Society of Arts," Adelphi, London. LITERARY NOTICES. JOHN CASSELL'S LATIN WORKS. The Latin Dictionary, in Numbers at 3d. and Parts 1s. each, by Dr. Beard, is now in course of publication, 3 numbers are ready, and the 1st Part will be ready with the Magazines for February. Secondly: From the vertex of the given angle, by Prop. XII., draw a perpendicular to one of its legs, and by the pre-s. Stoddard." This Grammar has been put to the test of experience, and "A Grammar of the Latin Language, by Professors E. A. Andrews and ceding proposition, make an angle equal to the angle contained pronounced by competent judges, who have brought it into use, to be a by this perpendicular and the other leg; but this angle is the production of superior merit. complement of the given angle; wherefore, an angle has been made equal to the complement of the given angle. Q. E. F.t "Cassell's First Lessons in Latin." This consists of a short and easy introduction to the Latin language, with Grammar, Exercise, and Vocabulary. It deserves the special notice of conductors of schools, masters under Government inspection, pupil teachers, self-instructing students, and all to whom cheapness, as well as excellence, is an object. "Lessons in Latin, by Dr. J. R. Beard." This volume contains an Elementary Grammar of the Latin language, in a series of easy and progresLatin, and Latin into English. For the benefit of those who are desirous of sive lessons; also, numerous Exercises for Translation from English into learning this language without a master, John Cassell has also published a Key to the above-mentioned Exercises, which, as well as the Lessons and Exercises, is by Dr. Beard. Cassell's Classical Library, Vols. 1 and 2, containing Latin Exercises and a Latin Reader, are both now ready. ERRATA. Vol. iii., p. 277, English-Greek, line 4, for plays read educates; vol. iv., p. 161, Greek-English, line 7, for flatter read are idle; English-Greek, line 4, for διώκουσι ad διώκονται, for Βλακεύουσι read Βλακεύουσιν. HENRY G. D. is partly right and partly wrong; the above will correct what is really incorrect. 2 ON PHYSICS OR NATURAL PHILOSOPHY. No. XVIII. (Continued from page 244). THE ATMOSPHERE. Amount of Atmospheric Pressure.-From the height at which the mercury becomes stationary in the Torricellian tube, at a mean state of the atmosphere, we can determine the amount of its pressure, in pounds avoirdupois, on a given surface. As the height is usually measured in inches, we shall first find the pressure of the atmosphere on a square inch. Now, supposing the area of the internal section of the tube to be a square inch, it is plain, by the rule for measuring the solid content of a cylinder, that the area of the base, one square inch, multiplied by the altitude, 30 inches, gives the solid content 30 cubic inches. Again, to find the weight of 30 cubic inches of mercury, we have this proportion, by the method explained at the end of Lesson XI., p. 158, viz., 1,728 cubic inches: 30 cubic inches 13,600 ounces: 236 ounces; so that 236 ounces, or 14 lbs. avoirdupois, is the weight which represents the pressure of the atmosphere on every square inch of surface. In this calculation, we have taken the specific gravity of mercury at 13 6; consequently, by multiplying this number by 1,000, we have, according to the rule above cited, 13,600 ounces for the weight of a cubic foot of mercury. The problem to find the atmospheric pressure on any given area at the surface of the earth, is now solved; for we have only to find the number of square inches which that area contains, and multiply it by 143 lbs., the measure of its pressure on a square inch; the product will be the number of pounds representing the pressure of the atmosphere on the whole area. According to some writers, the surface of the human body, taken of an ordinary stature and condition, contains 2,325 square inches; hence the mean pressure of the atmosphere supported by every man, on an average, is 34,294 lbs., or upwards of 15 tons weight! It would seem that, under such a pressure as this, we should be crushed to atoms; but our bodies resist its action by the reaction of the elastic fluids which it contains. Our limbs, also, suffer no pain in moving under this pressure, because, acting as it does in all directions, it supports us in every position, by pressures equal and contrary, and which, therefore, balance each other. Indeed, ou those days when the atmospheric pressure is the lightest, we suffer a species of uneasiness which makes us say the weather is heavy,” an expression exactly the contrary of what should be said. THE BAROMETER. This instrument, like all others of the same construction, is incapable of marking the variations in the barometric column with precision, because that the zero of the scale does not invariably correspond to the level of the mercury in the cistern. In fact, the pressure of the atmosphere being variable, this level varies whenever the pressure increases or diminishes; for then a certain quantity of the mercury passes either from the cistern into the tube, or from the tube into the cistern: so that, in most cases, the graduation of the scale does not indicate the true height of the barometer. The manner in which this cause of error may be avoided in the construction of a barometer will soon be explained. The height of the barometer is the difference between the level of the mercury in the tube and the level of the mercury in the cistern. As the pressure with which the mercury acts at the bottom of the tube is, according to the laws of fluids, independent of the form and the diameter of the tube, provided there be no capillary action, the height of the barometer is also independent of the same circumstances; but the height of barometers made of different liquids is in the inverse ratio of the density of the liquid. In the mercurial barometer, the mean height at the level of the sea, is 30 inches; in the water barometer, the mean height is 34 feet. Construction of the Barometer.-In the construction of barometers, mercury has been specially selected as the most convenient liquid, because of its having the greatest density, and consequently the least height in the tube; besides this, it has the property of being the least volatile of liquids, and it does not wet the glass of which the instrument is made. It is ot great importance that the mercury should be pure and free from oxidation; otherwise, it adheres to the glass and soils it. Moreover, if it be impure, its density is diminished, and the height of the barometer is too great. In every barometer, the vacuum at the top of the tube (figs. 70 and 71), which is called the barometric chamber, or vacuum of Torricelli, must be completely freed from air and watery vapour; otherwise these fluids would, by their clastic force, lower the mercurial column. This object is attained by first pouring into the tube a part of the mercury with which it is to be filled, and heating it to ebullition; this being allowed to cool, the rest of the mercury is then poured in and heated in the same manner, until the tube is full. Thus the air and humidity which adhere to the sides of the tube are expelled by the vapour of the mercury. In order to ascertain whether a barometer has been freed from air and humidity, give it a gentle inclination, and if it produces a dry and metallic sound, by the mercury striking against the top of the tube, it has been properly constructed; but if there be any air or humidity in the tube, the sound will be deadened, and the instrument inaccurate in its indications. The cup or cistern barometer is composed of a straight glass tube of about thirty-four inches long, filled with mercury and immersed in a cup or cistern containing the same liquid. Such, in fact, is the apparatus already described under the name of the Torricellian tube (fig. 69). With the view of rendering the barometer more portable, and the variations of level in the cup less apparent, when the mercury rises or falls in the tube, the form of the cup has been considerably varied. In fig. 71, a barometer of this kind is shown, which can easily be carried from place to place. The cup or cistern has two divisions, of which the largest is cemented to the tube, The Portable Barometer.—Another form of the cistern baroand communicates with the atmosphere only by a small meter has been constructed by M. Fortin, but it differs opening, having a lid made of leather, which is shown on considerably from the preceding one. The bottom of the the top of the cistern near the tube. Below the first com- cistern is made of deer-skin, and it can be raised or lowered by partment of the cistern, there is a smaller one, completely means or a screw placed below it; thus yielding the double full of mercury, the former being only partially filled. advantage of obtaining a constant level in the cistern, and of These two compartments are connected by a neck, or con- rendering the instrument more portable. For the purpose of tracted part, in which the lower end of the barometric tube travelling, it is sufficient to screw up the bag of deer-skin conis inserted. This end does not completely fill up the passage taining the mercury, until the cistern and the tube are combetween the two compartments; but it is constructed so as i pletely filled with it; the barometer may then be placed in any VOL. IV. 96 position, even an inverted one, without the danger of breaking the tube by the shaking of the mercury. This instrument is shown at fig. 72, where the tube is enFig. 72. Fig. 73. closed in a brass case for safety. In this case, there are two longitudinal slits exactly opposite to each other, through which the level of the mercury in the tube may be seen. On the case, there is placed a graduated scale, with divisions to the 25th part of an inch. A slide A, moved by the hand, and furnished with a vernier, gives the height of the barometer to the 250th part of an inch. In fig. 73, the parts of the cistern are shown upon a larger scale; it is constructed in the form of a glass cylinder, in order to show the level of the mercury in it. The bottom of this cylinder is closed by a piece of deerskin BD, which is raised or lowered by the screw C. This screw works in the bottom of a brass cylinder, to which the glass one containing the mercury is internally fixed so as to be completely protected from injury. In the top of the cistern there is fixed a small ivory rod A, the point of which corresponds exactly to the zero of a graduated scale, with divisions to the 25th part of an inch, marked on the case. In using this instrument, care must be taken, at every observation, to bring the level of the mercury in the cistern to this point, by turning the screw c' either way, as occasion may require. In this manner, the distance a o, fig. 72, represents exactly the height of the barometer. The Siphon Barometer.-The siphon barometer is composed of a glass tube bent into two unequal branches. Of these, the greater, which is closed at the top, is filled with mercury, like the cistern barometer; and the smaller, which is open, takes the place of the cistern. The difference between the level of the mercury in these two branches is the height of the barometer. In fig. 74 is shown the siphon barometer, as modified by M. Gay-Lussac, who, in order to render it portable, and prevent the admission of air into the tube, united the two branches by a capillary tube. When this instrument is inverted, the tube, in consequence of the capillary joint, remains always full, and the air cannot find its way into the greater branch, Still, if it be subjected to too sudden a shock, the column of mercury in the tube may be divided, and the air allowed to gain admission. To obviate this inconvenience, M. Bunten has adopted the modification shown in fig. 75. The capillary tube, instead of being fastened to the greater branch, is fixed to a Fig. 74. tube K, of larger diameter, into which that branch enters in the form of a tapering point. By this arrangement, if air-bubbles pass into the capillary tube, they cannot enter into the tapering point of the tube, but must lodge in K, the highest part of the tube of larger diameter, as shown in figure; there they cannot affect the operation of the instrument, as the vacuum always exists at the top of this part. In the barometer of Gay-Lussac, the short branch is closed at its upper extremity, and there is only a small lateral opening at c, which communicates with the atmosphere, and allows its pressure to take place. As to the measure of the height, this is effected by means of two scales, having their common zero at o, near the middle of the greater branch, and graduated in contrary directions, the one from o towards E, and the other from o towards D, on two brass rules parallel to the barometric tube. Two verniers are made to slide on the scales in such a manner as to indicate the number of 25ths of an inch, and tenth parts of the same, contained between o and A, and o and B. The sum of the two numbers thus obtained will be the whole height A B of the barometer. In fig. 74, the barometer of Gay-Lussac is represented as fixed on a mahogany board for the purposes of demonstration; but for travelling purposes, it is enclosed in a brass case, exactly like that of the portable barometer, only wanting the cistern. Correction of the Height.-In cistern barometers, there is always a certain depression in the height, arising from capillary action. In order to correct this error, we must know the interior diameter of the barometric tube, and then, by means of the table given in Lesson XIV., p. 204, we find the correction which must always be added to the observed height. In the barometer of Gay-Lussac, this correction is avoided by making the two branches A and B of the same diameter; for then the depressions at A and B being equal, the column A B preserves its true length. K Fig. 75. A In all observations made with the barometer, whether of the cistern or siphon construction, the temperature must be taken into consideration. For, since mercury expands or contracts by variations in the temperature, its density changes, and consequently its height; because this height, as before remarked, is in the inverse ratio of the density of the liquid contained in the tube; hence, for different atmospheric pressures, we might have the same height in the barometer. It is important, therefore, at every observation, always to compare the height with that which it would be at a fixed and invariable temperature. This being entirely arbitrary, we may adopt the temperature of freezing, or 32° Fahrenheit. The method of making this correction will be explained when the temperature of the mercury, a thermometer is placed near we treat of the subject of heat. For the purpose of ascertaining the tube, as shown in figs. 71 and 74. By a very simple calculation, the height of the barometer can be referred to zero; for it is only necessary to employ tables of correction which have been constructed for this purpose, Variations in the Height of the Barometer-When observations are made on the barometer during several successive days, it is found that its height varies in every place, not only day after day, but even during one and the same day. The amplitude of the variations, that is, the mean difference betweening to this coincidence between the height of the barometer and the greatest and the least height, is not always the same. It the state of the weather, the following indications have been increases from the equator to the poles. The greatest varia- marked on the scale of the barometer, above and below the tions, except in extraordinary cases, are about a quarter of an standard point. inch at the equator; 13 inch at the tropic of Cancer; 1% inches in latitude 45°, or about the mean latitude of France; and 2 inches at 25° from the pole, or in latitude 65° N. Moreover, the greatest variations occur in winter. The mean 'diurnal height of the barometer is found by taking the height every hour for 24 hours, and dividing the sum of these heights by 24. M. Ramond has observed that, in France, the height of the barometer at noon is almost exactly the same as the mean diurnal height. The mean monthly height is found by adding the mean diurnal heights taken during a month, or rather during 30 successive days, and dividing their sum by 30. 365. The mean annual height is found by adding the mean diurnal heights taken during a whole year of 365 days, and dividing by At the equator the mean annual height, at the level of the sea, is 29.84 inches. It increases between the equator and a certain limit, and reaches, between the latitudes of 30° and 40°, a maximum of 30.04 inches. It decreases in higher latitudes, and at Paris it is only 29.80 inches. The general mean height, at the level of the sea, appears to be 29.96 inches. The mean monthly height is greater in winter than in summer; and this arises from the coldness of the atmosphere. In the height of the barometer, two kinds of variations are observable: 1st, the accidental variations, which present no regularity in their occurrence, and which depend on the seasons, the direction of the winds, and the geographical position of the place; these are specially observed between the latitudes of 40° and 50°. 2nd, the diurnal variations, which are periodically produced at certain hours of the day. At the squator and in the inter-tropical regions, no accidental variations are observed; but the diurnal variations take place with such regularity that a barometer might be used as a sort of clock. After midday the barometer falls till about 4 p. m. when it reaches a minimum; then it rises and reaches a maximum about 10 p.m. Again it falls, reaches a second minimum about 4 a. m. and a second maximum about 10 a. m. In the temperate zones, there are also diurnal variations; but they are more difficult to determine than at the equator, because they are mingled with accidental variations. The hours of the maxima and the minima of the diurnal variations appear to be the same in all climates, and in any latitude; but they vary a little with the seasons. Cause of the Variations of the Barometer.-In general it is observed that the motions of the barometer take place in a contrary direction to those of the thermometer; that is, when the temperature rises the barometer falis, and vice versa. This fact indicates that the variations of the barometer, in a given place, arise from the expansions or contractions of the air in that place, and consequently from changes in its density. If the temperature of the air were constant and uniform, throughout the whole extent of the atmosphere, no current would be produced in its interior; and the atmospheric pressure, at the same height, would be invariable in every place. But when a certain region of the atmosphere is heated more than those in its vicinity, the expanded air rises in consequence of its specific lightness, and flows through the higher regions of the atmosphere; whence it follows that the pressure decreases and the barometer falls. The same effect would be produced, if any region of the atmosphere preserved a given temperature, and those in its vicinity were cooled down from that temperature; for then the air of the former would still rise in consequence of its less density. It also generally happens that an extraordinary fall of the barometer at any place on the globe, is compensated by a corresponding elevation at another place. As to the diurnal variations, they seem to arise from the expansions and contractions periodically produced in the atmosphere, by the calorific action of the sun's rays during the revolution of the earth on its axis. Relation between the Barometer and the Weather-It is observed that, in our climate, the barometer commonly stands at the height of 30 inches; that it falls below this point when there is rain, snow, wind, or storm; and that, when for a certain number of days the barometer has stood at 30 inches, there are, at a mean, as many days of fine weather as there were days of rain. Accord Height. 28.0 inches 28.5 29.0 State of the Weather. stormy. very rainy, or snowy. fair, or frosty. In consulting the barometer in reference to the changes of the weather, it must not be forgotten that this instrument is only intended to measure the weight of the air, and that it only rises and falls as this weight increases or diminishes. Now although the changes of the weather most frequently take place at the same time with the variations in the pressure of the atmosphere, it does not follow that they are invariably connected. The winds which come to Europe from the south-west, being always the warmest, and consequently the lightest, cause the barometer to fall; but as they are at the same time charged with watery vapour by traversing the ocean, they also bring rain. The winds which come to it from the north and the north-west, being on the contrary, the coldest, and consequently the densest, cause the barometer to rise; but as they come across vast continents to our climates, they are dry or freed from watery vapour, and e generally accompanied with a clear and serene sky. The warm winds of the south-west tend to increase the pressure of the atmosphere by the weight and tension of the vapour which they contain; but at the same time they tend to diminish it by their expansion. The latter cause being the most powerful, the ultimate result is, that owing to the elevation of the temperature, the winds in our climate cause the barometer to fall. At the |