350 A.CROPHYSICS. 



the water ; which is therefore forced upwards through the valve of 

 the piston. In the lifting, and forcing pumps, the valve of the 

 piston opening in one direction, allows the fluid to pass, and then, 

 by the opposite motion of the piston, forces it onward, without allow- 

 ing it to return. The air pump, is a forcing pump, with tight pistons 

 and valves, for exhausting the air from any attached vessel. The 

 barometer, consists of a glass tube, more than 30 inches long, open 

 at one end, which dips into a cup of mercury exposed to atmospheric 

 pressure. The mercury with which the tube was at first entirely 

 filled, descends in it, leaving a vacuum above, but still remains about 

 30 inches higher than that in the cup ; varying with the change of 

 pressure of the air. Its use, in calculating heights, depends on the 

 falling of the mercury, about T ^ of an inch for every 90 feet in height 

 that we ascend. 



The atmosphere would rush into a vacuum, at the level of the sea, 

 with a velocity of about 1330 feet per second. The resistance of 

 the air to bodies moving through it, increases with the square of the 

 velocity ; as in the case of liquids ; so that by doubling the velocity 

 the resistance is increased fourfold. Hence, vessels moving swiftly, 

 are propelled at a great sacrifice of force. Sound, is caused by 

 vibrations of the air, produced by sounding bodies : but though the 

 sound moves onward to a great distance, each particle of the air only 

 vibrates through a very small space, towards and from the source of 

 sound. The rapidity of these vibrations, determines the note, or 

 tone; as will be explained in treating of Music. The vibrations are 

 propagated successively, and continuously, like waves from a pebble 

 thrown into still water : and hence we say that sound moves at the 

 rate of about 1130 feet per second, in the air; and nearly ten times 

 as fast, along cast iron. 



H CHAPTER II. 



ASTRONOMY. 



ASTRONOMY is that branch of Acrophysics, or Natural Philosophy, 

 which treats of the heavenly bodies ; and their relation to the earth, 

 regarded as a part of the planetary system. The name is derived 

 from the Greek, acr^pov, a star, and fo/*o$, a law ; literally signifying 

 the laws of the stars. The most prominent heavenly bodies, compose 

 a central group ; central at least so far as the universe is known to us, 

 and called the solar system ; at an immense distance from which are 

 \hejixed stars ; the nearest of them being at least 200,000 times as 

 far from us as we are from the sun. The solar system, consists of 

 the sun, placed at its centre ; the planets, of which the earth is one, 

 revolving around it in nearly circular orbits ; the satellites, revolving 

 around the planets ; and, lastly, the comets, which also revolve around 



e sun, but in very eccentric orbits. Could we fly at the rate of 

 100 miles an hour, the speed of the tempest, we should require more 

 than twenty million years to reach the nearest fixed star : and our 



