These relative differences are the same in Norway and in Lapland, although the absolute heights may be different. Thus, if we see the pines disappear at 980 metres, we shall find the birch disappear at 980+245=1225 metres; and the line of perpetual snow will be at 1225+578=1803 metres. The absolute height of these lines, will depend, not merely on latitude, but on the vicinity of mountains, their height, and their extent. The line of perpetual snow is lower in the neighbourhood of extensive chains of mountains, than on the sides or in the vicinity of a solitary peak. The line of perpetual snow also depends, chiefly perhaps, on the temperature of those months during which the snow might melt; it does not depend merely on mesne temperature. Thus, in the interior of the Gulph of Alten, the the mesne degree of the thermometer is less than at North Cape, yet the line of constant snow is more elevated. This happens, because an annual mesne temperature may result from mesne monthly temperatures very different. Thus, at Mageroe in lat. 71, and at Uleoberg in lat 65, the mesne annual temperatures differ but little, the first being +0,03 and the last +0,63. But at Uleoberg the summer months are greatly warmer than at Mageroe, although the winter months are colder. Hence the line of perpetual snow, being regulated chiefly by the temperature summer months, becomes in some sort, a measure of the force of vegetation, which must of necessity depend on the temperatures above the freezing point.

The line of perpetual snow in different latitudes has been profoundly discussed in Humboldt's Prolegomena de distributione geographicâ plantarum. His conclu

sions on this subject (on isothermal lines) are briefly these:

1st. Between the tropics from lat. O. to 10. among the Cordillieras of the new world, the limit of perpetual snow is at 4796 metres, or 2460 toises, (15735 English feet nearly.) The mean temperature of the air at this height, is not zero, as Bonguer and some other observers after him have fixed it, but at 11⁄2 of the centigrade thermometer.

2. Between the latitudes of 19 and 21 North, at Mexico, at the commencement of the torrid zone, perpetual snow is found, at 4580 metres, or 2350 toises, (15026 feet English.)

3. Under the temperate zone, at Caucasus in lat. 42 and 43, the height of this line, according to M. M. Engelhardt and Parrot, is at 3216 metres or 1650 toises.

4. In the Pyrenees lat. 42 to 43, M. Raimond found the snow permanent at 2729 metres, or 1400 toises. At this height, the mesne annual temperature is 3,5. (three and a half degrees below zero of the centigrade thermometer.)

These differences in the heights respectively at which the line of perpetual snow is found, depend

on

the circumstances already noted by M. Buch of the height of the mountains, the extent of the chain, and the mesne temperature of the summer months.

5. The mesne (average) of the observations recently published by M. Wahlenberg, gives for the line of snow in the Alps, lat. 453 to 46, 2670 metres or 1370 toises. At this height, the annual temperature is 4. The mesne of winter temperature is 10; that of summer + 6.

6. The mesne temperature of the year, at the height where M.

Buch found perpetual snow in lat. 68, is 6. That of the winter 201, that of the summer+91. From the parallel of lat. of Popocatapec in Mexico, to Etna, the line of perpetual snow has not been determined by direct observation. It follows from the researches of M. de Humboldt that this limit at the Peak of Teneriffe, in lat. 28°. 17'. ought to be 3800 metres or 1950 toises; but the height of this mountain is only 3711 metres or 1904 toises, so that if the Peak of Teneriffe is free from snow during summer, this does not arise from any effect of volcanic fires within the bowels of the mountain, but want of height. It may be curious to trace the progress of this branch of science of late years, by comparing the heights of Kirwan, with those of actual observation.

Journal of Science and the Arts, No. 3, well deserves an attentive perusal.

Mr. Daniel Wilson of Dublin has contrived a new hygrometer, which promises to be more accurate and delicate than those heretofore in use. He takes the urinary bladder of a rat, which is a small, stout, spherical body; and ties it firmly to the lower extremity of a thermometer tube. The thermometer is then filled with mercury; so that when the bladder is exposed to a perfectly moist atmosphere the mercury stands near the bottom of the tube. This point is marked zero. The instru ment is now suspended in a glass vessel together with a quantity of strong sulphuric acid, so as to render the atmosphere around it as dry as possible. The dimensions of the bladder somewhat diminish, in consequence of which the mer. cury rises in the tube. The point at which it remains stationary is marked 100°, and the distance between 0 and 100 is divided into 100 equal parts or degrees; so that O on this instrument, denotes extreme moisture. and 100 extreme dryness. Mr. Thompson proposes to reverse the scale, by placing O at the point of extreme dryness and 100 at the point of extreme moisture. This instrument is so delicate that the approach of the hand makes it sink several degrees. Mr. Wilson has made comparative experiments with these instruments for more than a year, during which time they did not alter their nature, but corresponded correctly with each other at the end of the time. Mr. Wilson has patented this invention. Thomps. Ann. Aug. 1816, p.

154.

OPTICS.

"When light falls upon a body of a sombre hue it is partly absorbed; but when it falls upon a white substance, or a polished surface, it is more or less completely reflected. The angle of reflection is equal to the angle of incidence. The reflection of light is variously modified by the forms of the surfaces from whence it arises; as from convex, concave, cylindric, and other mirrors.

"Some diaphanous bodies possess the property of dividing the ray of light which traverses them into two points, one of which follows the law of ordinary refraction, and the other a particular law, which was discovered by Huyghens.

"Transparent carbonate of lime exerts this action in a high degree. The angle of ordinary refraction always bears a ratio to the angle of incidence: the angle of extraordinary refraction depends upon the direction of the ray with regard to the axis of refraction (a line which coincides with the axis of crystallization in carbonate of lime.) When the ray is directed in a perpendicular or parallel direction to this axis, there is no extraordinary refraction; but when it is inclined to it, the refraction is greater or less, according to the angle of inclination.

"Light thus refracted is endued with some particular properties. When it is again made to pass through a rhomboid of double refracting spar, whose axis is parallel to that of the original crystal, it passes on without suffering any division: but if the second rhomboid be turned slowly round while the first remains stationary, each of the pencils begins to separate into two: and when the eighth part of a revolution is completed, they ar

rive at their furthest point of division: when the fourth part of a revolution is effected, the pencil refracted in the ordinary way by the first crystal is wholly refracted in the extraordinary way by the second; and that refracted in the extraordinary way by the first, is ordinarily refracted by the second, The same phenomena occur at every quadrant of the turn. Light which possesses these properties is called polarised light, and its peculiarities are supposed to depend upon a peculiar relative arrangement of its particles, in which their axes and similar faces are all similarly disposed.

"This modification is not conferred solely by refraction. Malus has discovered, that light reflected from various substances at certain determinate angles for each, is endued with the same properties. This angle in glass is 35o.*

"Polarised light is affected in a particular manner by reflecting surfaces. When a second reflecting plane is placed parallel to the first, the ray is wholly reflected; but when the new plane is perpendicular to the original one, it is, on the contrary, entirely refracted. The intermediate degrees are characterised by intermediate quantities of absorption and reflection. Polarization may also be conferred by ordinary refraction. Thus, in passing through glass, light is polarized in part; and if we transmit it through a series of parallel glasses, part of the molecules which have escaped the operation of the first are detained by the second, and another portion by the third: so that at last, if the number be sufficient, a completely polarized ray is obtained.

* 35° 25′. T. C.

"There is another modification of light which is amongst the recent discoveries of the present day. It is supposed to arise from an oscillation of the particles around their centres of gravity. If a ray of polarized light be made to pass through a thin leaf of mica, or selenite, and then analysed by a rhomboid of double refracting spar, it no longer passes through single, but two images are produced, of different colours, which are complementary to each other, that is to say, which produce white light by their mixture. The ray which falls upon the mica penetrates entire to a small depth, without the axes of its particles experiencing any deviation from their position; but at a certain depth, which is different for the different coloured particles, they begin to oscillate like the balance of a watch. These oscillations are confined to the same limits, but vary in velocity. The violet particles turn more rapidly than the blue, they more rapidly than the green, and so on to the red, which are the slowest of all. From this inequality it happens, that for every thickness of the leaf, different colours are found at the two limits of oscillation; and from hence arise the two differently coloured pencils, which are observed in analysing the transmitted light,

Cours elementaire et generale des Sciences Physiques.

When a ray of light enters a crystal whose primitive form is neither a regular octahedron, or a a cube, it is generally observed to be divided into two bundles or fasciculi unequally refracted. One is termed the ordinary fasciculus which follows the law of refraction discovered by Des Cartes, and which is common to all bodies whether crystallized or not; the other, which is termed the extraordinary fasciculus, follows a different and more complicated law.

Huygens determined this last law, by observations on the double refracting spar, Iceland chrystal, or diaphanous rhomboidal carbonat of lime. La Place combining this fact, with the general principles of mechanics, deduced a general formula for the velocity of the luminous particles of the extraordinary fasciculus. This formula, indicates that the particles of light are separated by a force emanating from the axis of the chrystal, which in the double refracting spar, is repulsive.

Malus however, may be considered as the first to whom we owe the modern ideas of the polarization of light, since pursued with much success by Biot, Arrago, and Pouillet in France, and Mr. Porret, and Dr. Brewster in Great Britain.

The above is an extract from Mr. Brande's account of Beudant's

(its refraction and diffraction) in its passage through various transparent substances, or coloured fringes, and on the phenomena of its reflection from glass and metallic mirrors, within this twelve month have been very numerous.

It is deducible generally from the facts announced, that light in