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standard instead of water, the weight of which is about one eight-hundredth part of the former: therefore, as a cubic foot of water weighs 1000 ozs., the weight of a foot of air will be 106 or 1.25 oz.; ten feet will be 12.5 ozs., 100 feet 125 ozs., etc. ; then having the specific gravities of other gaseous substances, some of which are heavier, some lighter, than the atmosphere, they may be made to calculate the weights of given volumes.
The principle of the thermometer should be explained how it is made-how graduated - and how the freezing and boiling points are determined -- why the tube is of a narrow bore, etc.
In the Boys' school at Somborne there is a barometer and a thermometer, which they are in the habit of observ. ing; registering the height when they go in, and noticing the course of its rise from increased temperature; this is registered three times a day, and a thermometer kept in the open air. The height of the barometer—the taking a weekly and a monthly average-forms an exercise of their arithmetic.
Attention might be called as to how such averages of the thermometer are affected by swampy and marshy grounds of great extent-improved drainage * -how this is likely to affect the temperature of a district, so much so, even as to advance the period of harvest – how the height of the thermometer may be affected by particular aspects
whether the line of country slopes towards the north or south, or is a level plain, etc.
The subject of heat t is one of great interest, and one on which the teacher may bring to bear a variety of experiments not attended with much expense, and having this additional recommendation, that they have an intimate relation with many of the comforts and conveniences of
* I was told by an experienced farmer in the county of Cambridge, that he believed the average period of harvest in that county was earlier by ten days, within the memory of man, owing to improved drainage.
+ The volume on Heat in Lardner's “Cyclopædia” will be found a very useful book for the schoolmaster, and as an introduction to practical science for pupil teachers.
Heat is present everywhere and in every kind of matter: we cannot measure its quantity ; but we can measure the quantity in one body relatively to that of another.
The general effect of heat upon matter is to expand it, that is, an increase of heat in the same body produces an increase of volume, in some proportion to the increased temperature.
This increase of volume for a given increase of temperature varies in different kinds of matter ; air and gases expand most, fluids next, and then solids.
Instances of each have been mentioned — as a full kettle swelling and flowing over before it boils - a round piece of iron fitting exactly into a ring when cold, when heated is too large.
Then, again, heated bodies impart heat to every thing around them until all have acquired the same temperature; as the heater for a box-iron for ironing linen, when put into the fire becomes red hot like the cinder; when taken out it is put into the box, communicates heat to it, and so to the linen ; and, when used for a certain time, becomes of the same temperature with the things around it.
We call things which we touch, hot or cold, according as they are hotter or colder than the human body, but in this the sense of touch deceives us; when we touch a body hotter than the hand, we receive heat from it-when we touch one colder than the hand, it receives heat from us; but experience tells us that all the things in a room, when measured by a thermometer, have an equal temperature, yet they do not feel equally so to the hand.
The different degrees in which bodies conduct heat have been ascertained by experiment; air and gases, when con. fined, are very bad conductors ; metals varying in degree among each other are good ones-generally the more dense the body, the better conductor it is.
Porous bodies are bad conductors, as are any bodies which contain air confined in cells, such as the feathers of birds -- the fur of animals— the bark of trees. All these how beautiful a provision for the preservation of animal and vegetable life!
Then, again, straw, reeds, etc., are bad ones; so that a
thick covering of thatch is a much better covering for a cottage, so far as warmth in winter and coolness in summer are concerned, than either tile or slate.
Tile, being rather a thick and a porous substance compared with slate, is better than the latter; and every one who is in the habit of visiting the cottages of the poor will have observed that the bedrooms of those covered with slate are in the summer extremely hot, and in winter equally cold.
Slate, again, would be better than iron.
The teacher would do well to observe the variety of fur and hair in animals, varying with the climates they inhabit; in warm climates the hairy coat of animals being short and thin, in the colder ones becoming thick and woolly. The birds of colder regions that live in the air, have a much greater quantity of plumage than those of the warmer ones ; water.fowl, such as ducks, geese, etc., have the interstices between their feathers filled up with down, more particularly on the breast. In the cold weather in winter, the birds may often be seen shaking and ruffling up their feathers in order to increase the quantity of air among them, which, being a bad conductor, helps to keep them warm.
Earth is a bad conductor, and the sharpest frosts in consequence scarcely ever get more than a few inches deep into the ground. The temperature of the earth, a very little below the surface, is the same in every climate. ,
In covering up a potatoe-pit for the winter, the lighter the soil, and the more of a covering of straw or leaves between it and the potatoes, the better they will be preserved. When it is said the frost gets to the potatoes, the thing really meant is, that the temperature of the air becoming lower than freezing point, the surface-covering of the potatoe-pit first gives out heat to the air, then that nearest the surface to the particles adjoining, until, last of all, the potatoes give out heat to what is resting upon them, and so the water of the potatoes gets cooled below freezing and becomes solid, and the potatoe spoiled;—hence the necessity of covering them with bad conductors--not to make the soil over them a solid, but as light as possible.
On the same principle, a covering of snow is a great protection, in very severe frosts, to the more delicate plants; although the temperature may be very far below the freezing point, and in some climates where the cold is great, the thermometer is even down to zero, yet the temperature of the ground, under a covering of snow, would be very little below freezing. Thus water in pipes below the surface, and in springs is never frozen. In the winter, to prevent water freezing in pipes which are above ground, they are wrapped round with straw or some bad conducting substance, etc. Ice-houses with double walls-rooms with double windows are all instances of the same kind. The application of a kettle-holder, having wood or ivory handles to teapots made of metal, etc., belong to the same principle.
The following, by way of a lesson on one of the metals, iron, with the experiments which follow, will convey some idea to the teacher of the mode of proceeding here, and may serve as a model for the way in which he would treat the other metals :
Iron— found in the earth as a mineral-how obtained from the ore?- is a metal a solid ? can it be made fluid ? Yes, sir, by great heat. Have you ever seen it fluid? - At the little foundry at the blacksmith's shop.--How does it become solid again? By cooling. - What effect has heat upon metals ?* It expands them, makes them longer-it would make an iron ring larger.-Have you ever seen this property of expanding by heat turned to a useful purpose? Yes, sir; the village blacksmith hooping wheels; he makes the hoop a little too small, heats it red hot, which makes it larger, and it just fits the wheel-he then pours water upon it; it immediately contracts and makes the joints of the wheel close up and crack, and so it fits tight-riveting bolts, etc.—the experiment of iron bars bringing the opposite sides of a building to an upright position from leaning outwards.
Į am told, in testing the anchors in the dockyard at Portsmouth, that the largest anchors have a strain on them of perhaps 150 tons, and being in length about 30 feet, and
* See p. 215.
as thick as the body of a man, that immediately the strain is taken off they will collapse as much as an inch, and that this shrinking is visible to the eye of a looker 06.
A bar (whose length at 32° is taken at unity) of the following substances will, when heated to 212°, the boilingpoint of water at the ordinary pressure of the atmosphere, expand; glass, ilio of its length; steel about gó7; iron, gir; copper, z'gi silver, 314; tin, ąáz; lead, 3}t; or a rod of iron whose length (temperature 329) is 846 inches, will, at the heat of boiling water, expand one inch, and become 847; tin, length 462 inches, would become 463.
The difference between the heat of summer and winter will cause such a variation in the length of the ordinary seconds pendulnm as to affect its time of vibration; and in the building of iron bridges, allowance is obliged to be made for what is called the play of the iron, between summer and winter heat, or the whole would come down, and I believe in some of the large tubular structures of iron lately erected over rivers, allowance has been made for theun. equal expansion of the metal on the sunny and shady sides.
The teacher will point out the various uses to which iron can be applied-how useful from its extending under the hammer-Welding (which most other metals do not), and other properties. What is welding? Heating two pieces of iron to a very great heat (called a white heat), then placing them together on the anvil, and beating them with a hammer, they unite as one piece; silver and gold will not do this. Platina welds.
Cast-iron-melted and run into a mould for shape, for grates, saucepans, boilers, teakettles, part of the plough, rollers, door-latches, gate-latches.
Did you ever in winter, in frosty weather, find out that it was colder to the hand to touch iron than wood ? Yes, sir.- Why? Do not know, sir.-Teacher (making the children touch substances of different conducting powers, a piece of marble, stone, wood, wool, fiax, cotton, etc., pointing out to them that all have the same temperature as the room, which is below that of the hand, and ought, so far as this is concerned, to affect it equally): Because iron is a better conductor of heat than wood or any of the others: