cold, first observed by the Italian philosophers, and afterwards by Pictet. If in the arrangement of the two paral lel mirrors, a piece of ice be introduced into the lower focus, the thermometer in the upper focus will indicate a diminution of temperature.

579. Rainbows arise from the rays of the sun which fall on drops of water being reflected and refracted to the eye of the spectator; and, of course, all those drops which are situated at t'e same angle all round from the eye, will present the same colour.

And, as different colours will arise at different angles; a bow composed of regular circles is a necessary consequence of showers of rain, while the sun shines.

It will, however, only be visible opposite to the sun; and the line from the sun through the eye of the spectator must be its centre.

580. As rays will reach the eye from drops of rain, owing to two different causes; so there will generally be two rainbows, one fainter, however, than the other.

The strongest, or lowermost rainbow, is occasioned by the light being reflected from the upper part of the back of the drops of rain; and the other, or upper bow, is occasioned by the light being twice reflected within the drop from the lower part to the upper, and thence, refracted to the eye.

581. The breadth of a rainbow is about two degrees; and, of course, no two spectators can see the same rays; but every eye will be the centre of its own bow.

All circles round the sun and moon, arise in like manner, from the peculiar modifications of

the rays passing through the vapours of the atmosphere in peculiar states.

Obs. 1.-The rays of light are shewn as passing from the sun to the drops of water, and thence to the eye of the spectator E; and all the rays at the same angle from the eye or centre P, necessarily produce the same colours.

2.-The spectator stands, of course, with his back to the sun, and his eye is necessarily the centre of the bows, each drop of the same colour having an equal angle from the eye. Rainbows, of course, are more or less vivid, as the sun shines more or less bright on the opposite rain; and they are more or less perfect, as the rain is more or less diffused. An artificial bow may be made with a fountain; and glass chandeliers reflect colours on exactly the same principles.

3. In the inner Bow, the colours are red at top, then orange, yellow, green, blue, indigo, and violet; and, in the upper Bow, the contrary. The upper Bow makes an angle with the eye of 54° and 51°; and the lower, of 420 and 40°. The centre of the circle is a line passing from the Sun, through the eye of the spectator.

582. A magic lanthorn is founded on the principle of placing the image within the focus of the lens; so that the rays diverge and produce a figure as much larger as is desired on a wall; and Phantasmagoria are produced by magic lanthorns; in which all the parts of the sliders, except the figures, are painted black and opake.

The camera obscura, for drawing landscapes, consists merely of one lens, with a mirror to reflect the images on the rough glass placed to draw upon with a pencil.

Obs. 1.—Before I leave this subject, I must recommend the tutor or student to dissect a bullock's or sheep's eye. By taking off the back delicately, he will see the landscape before the eye beautifully painted on the optic nerve; he will find in front, the cornea; beneath it, the aqueous humour; then the iris, so called from its various colours; the pupil or hole, which opens and shuts to the light; and the chrystalline humour, or lens. Then, the vitreous humour; and then, the optic nerve or net-work; -a curious and wonderful arrangement!

2.-The instantaneous motion of light has given rise to the TELEGRAPH, a modern invention of the greatest social importance, at present limited to purposes of governments; but capable of the highest uses to the community at large.

The telegraph consists of a large frame, in which are placed and worked six shutters, marked a, b, c, d, e, f, by means of ropes pulled in the manner of bell-ropes. By the various combinations of these shutters, 63 distinct signals may be produced, sufficient to represent the 24 letters of the alphabet, the ten digits, and various leading words. Such telegraphs are then set up on eminences at the distance of 8, 10, or 12 miles; and a line of them, by repeating each other's signals, conveys a message from the first station to the last, at the rate of a hundred miles in about five minutes!

The hole and telescope at T, is for the observer; who, in clear weather, is constantly on the look-out.

XXVI. Meteorology.

583. Every 100 parts of the atmosphere, is composed of 28 parts of oxygen, and 72 of azote or nitrogen, kept in a gazeous state by caloric or heat.

Atmospheric air is found to weigh in proportion to rain water, as 12 to 10,000; it is to oxygen gas, as 5 to 6; and to hydrogen, as 15 to 1; a cubic foot of it, weighing 525 grains; or one ounce and a quarter, nearly.

584. The atmosphere is found to be very elastic; and, in consequence, to press on every side, equal to a weight of 33 feet of water, or 294. inches of mercury; and this elasticity is found to decrease, as we ascend higher and higher, so as to render the barometer a means of ascertaining heights.

Obs. 1.-This elasticity is equally powerful in a cubic inch of the atmosphere, as in the whole mass; and an inch will raise the mercury in the barometer, as much as the whole atmosphere. One cannot, therefore, but wonder at the quackery, or inconsiderateness of authors, who copy, one after another, the idle nonsense about the atmosphere pressing a man with a weight of 30,000 lbs.; when, in fact, he is not pressed to the amount of an ounce; all the vesicles of his body being filled with air, which presses outward, at least as much as the atmosphere presses inward, and also upward as well as downward. In fact, in regard to animal and vegetable bodies, the slight gravity of the air is destroyed by its elasticity.

585. Comparing the atmosphere to fleeces of wool laid upon one another, it will be lighter or rarer as we ascend in it; or, in other words, its elasticity will be diminished.