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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.
Obs. 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 of exactly the same principles.
Obs. 2.- 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 54o and 51°; and the lower, of 42° and 40°. The centre of the circle is a line passing from the Sun, through the eye of the spectator.
598. 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 łens, 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 bellé ropes. By the various combinations of these shutters, 63 distinct signals may be produced, sufficient to represent the 24 letters of the alphabet, the 10 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.
XXVII. Meteorology. 599. Every 100 parts of the atmosphere is com posed of 28 parts oxygen, and 72 of azote or nitrogen, kept in a gaseous state by caloric or heat.
Atmospheric air is found to weigh in proportion to rain water, as 12 to 10,000, and to oxygen gas, as 5 to 6; and to hydrogen, as 15 to 1; a cubit foot of it weighing 525 grains ; or one ounce and a quarter, nearly.
600. 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 291 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.
601. 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.
Were it all of uniform density, like water, it would be about five miles high ; but the reflection of the sun's rays appears to be affected by it at the height of 44 miles ; where it is calculated to be 4,000 times less elastic than on the surface of the earth.
The blue colour of the atmosphere is its natural colour. Its power of reflection produces the universal diffusion of light.
Obs.-On the elasticity of air, is founded the invention of the Diving BELL; by means of which, an operator descends to any depth in water, and remains there for hours together. Weights are placed at bottom to prevent it from turning ; and a forcing pipe sends in fresh air, to supply the waste of air from the respiration of the operator.
602. By means of the air-pump, all the air may be drawn out of a large glass-vessel, and a vacuity or vacuum produced ; in which a great number of curious experiments may be performed, shewing at once the properties and usefulness of air.
Che figure represents an air-pump on the best modern construction. Che glass-receiver, as it is called, is placed at top; where there is a bole, to let out the air by the action of the pistons seen below.
603. In the aerial vacuum, a feather and a guinea will fall with equal velocity, owing to there being no resistance of the air.
If a bladder, apparently empty, be tied at the neck and left in it, the small quantity of air in the bladder will swell it, and presently burst it.
A bell will cease to sound in vacuo.
The smoke of a candle, having no air to float in, will fall to the botttom by its own weight.
No animal will live, or any light burn, in vacuo.