PNEUMATICS. 



But there is a still more conclusive 

 argument that it is the weight of the 

 atmosphere which presses down the pis- 

 ton. If, by a valve in the bottom of the 

 cylinder, the air be admitted below the 

 piston, it will no longer be pressed 

 down, or rather it will be pressed both 

 upwards and downwards by equal forces, 

 and will be indifferent as to its ascent 

 or descent, except so far as the weight 

 of the piston itself will produce the 

 effect. This is owing to a property of 

 air, by which it presses equally in every 

 direction, which we shall explain more 

 fully hereafter. (13.) 



The most direct proof, however, that 

 aii- is a heavy substance is, that it can 

 be directly weighed. 



Let a phial be provided, containing 

 not less than two quarts, and having a 

 stop-cock screwed upon its neck. By 

 means of the air-pump or exhausting 

 syringe, which will be described here- 

 after, the air may be withdrawn from this 

 vessel. "When this has been done, and 

 the stop-cock closed, let it be suspended 

 from one arm of a very accurate ba- 

 lance, and an exact counterpoise placed 

 in the opposite scale-pan. The empty 

 phial is thus balanced. When the beam 

 ceases to vibrate and becomes steady, 

 open the stop-cock and admit the air 

 into the phial. It will immediately pre- 

 ponderate, and it will be found that to 

 restore the equilibrium a weight must 

 be placed in the opposite pan, at the rate 

 of about 523 grains for every cubic foot 

 of air contained by the bottle. Since 

 there are 1 000 ounces in a cubic foot of 



product will be the number of square inches in the 

 circle. 



This rule will give the area of the circle to within 

 one 10,000th part of the square of half its diameter. 

 The following example will serve to show the appli- 

 cation of this-, rule. Let the diameter of the circle be 

 21 inches. The square of half this is 12x12=144. 

 Hence the area will be found thus : 

 144 

 3.1415 



452.3760 



which expresses the number of square inches in the 

 circle to within one hundred aud forty-four 10,000th 

 of an inch. 



The area may be found without decimals by the 

 following rule : Let the diameter of the circle be 

 squared, and its square divided by 14. If the quote 

 be multiplied by 11, the product -will be the area 

 nearly. Thus, in the preceding example, the diame- 

 ter i 5 24, the square of which is 576; this, divided 

 by 14, gives 41 i, which, being multiplied by 11, 

 mtft 452 . 



water, it follows that, bulk for bulk, 

 water is about 840 times the weight of 

 air. 



(11.) Many effects with which we are 

 familiar, and which often excite our 

 curiosity, are accounted for by the gra- 

 vitation of the atmosphere. If the 

 nozzle and the valve-hole of a pair of 

 bellows be stopped, it will be found that 

 a very considerable force will be neces- 

 sary to separate the boards. This is 

 owing to the air not being permitted to 

 enter at the usual apertures, to resist the 

 pressure of the atmosphere on the ex- 

 ternal surfaces of the boards. Shell- 

 fish which adhere to rocks, snails, and 

 other animals, have a power by muscu- 

 lar exertion of expelling the air from 

 between the surface of the rock and the 

 surface which they apply to it, in conse- 

 quence of which they are pressed upon 

 the rock by the atmosphere with a force 

 of about fifteen pounds for every square 

 inch in the surface of contact. The 

 same cause enables flies and other ani- 

 mals to walk on a perpendicular plane of 

 glass or on the lower surface of an ho- 

 rizontal plane, apparently suspended by 

 their feet, and with their bodies down- 

 wards. This has lately been proved to 

 arise from a power of expelling the air 

 from between their feet and the~ surface 

 on which they tread, so as to obtain a 

 pressure from the atmosphere propor- 

 tionate to the magnitude of the soles of 

 their feet. 



CHAPTER III. 



Of the Weight of the Atmosphere The 

 Barometer. 



(12.) Having in the last chapter con- 

 sidered in a general way those proper- 

 ties which elastic fluids have in common 

 with even- species of matter, we shall 

 now examine more particularly the 

 weight of the atmosphere and the me- 

 thods of measuring it. It will be neces- 

 sary first, however, to mention a qua- 

 lity of all fluids, whether elastic or ine- 

 lastic, to which we shall have occasion 

 to allude. 



(13.) One of the most striking proper- 

 ties by which fluids are distinguished 

 from solids, and that indeed which has 

 been adopted in mechanical science as 

 the definition of a fluid, is the quality by 

 which it is capable of transmitting pres- 

 sure equally in every direction. 



To explain this, let us suppose a ves- 

 sel of any shape completely filled with a 

 fluid and closed at every part, so that 



