riIII.nS.irHY. I'NKT M \TI<'*. 



[rntt s 



tho surface of each planet ; and, more especially as 

 rrgiu i-scnee would become manifest 



by astronomical observation. But astronomers tind the 

 moon to be quite destitute of an atmosphere : it ha* no 



K m> rain ; and son innnta eem to be in 



the aarne prwliciun-nt. That the moun has no atmo- 

 sphere, iin.I . ..ii-.-.,u.-iii!y can have no breathing inhabit- 

 ants, is a fact interesting in itself ; and it involves this 



: fact, equally interesting, th.it our atmosphere has 

 iu limits. Accurately to assign these limits is beyond 

 the power of science ; but the phenomenon of the re- 

 fraction and reflection of light leads to the conclusion 

 that it extends to tlio height of about forty or fifty miles. 

 The average between these, namely, forty-five miles, is 

 usually regarded by philosophers to be the height of our 

 atmosphere. But that this conclusion may rest upon 

 something better than mere conjecture, we shall give the 

 reasonings by which it lias been arrived at. 



inospherical refraction is caused by the bending of 

 tin- rays of light from a luminous body upon entering 

 our atmosphere : till the outer boundary of the atmo- 

 sphere is reached, nothing diverts the direction of a 

 luminous my. This refraction of the rays of the sun 

 adds intensity to the twilight, or the light we enjoy after 

 sunset. The twilight is found to continue till the sun 

 is 18 below the horizon. Let, therefore, A H (Fig. 192) 

 be the horizon of an observer at A, who will continue to 

 Fig. in. 



have twilight till the ray S H from the descending sun 

 makes with B H the angle S H B = 18, and therefore 

 the angle A H S is 162. 



The last glimmer of twilight is due, no doubt, mainly 

 to the reflection of the ray S H from the particles of the 

 atmosphere at H ; and the radius C H being perpendicular 

 to the reflecting surface at H, the angles A H C, S H C 

 must be very nearly equal, and therefore each equal to 

 about 81. Now taking the radius C A of the earth at 

 4,OUO miles, we have by Trigonometry, 



C H . C A cosec.H - 



C A 4IHKI 

 -- - - - 4050 nearly. 



So thai, according to this calculation, the height h II of 

 the atmosphere is 4,050 4,000 = 50 miles. 



The height of an homogeneous atmosphere that would 

 press, as the actual atmosphere is found to do, may bo 

 readily ascertained. Thus, if we take the specific gravity 

 of air to that of water as 1 to 850, when the barometer 

 stands at 30 inches, and the specific gravity of water to 

 that of mercury as 1 to 14, we shall have the specific 

 gravity of air to mercury as 1 to 11,000. 

 .'. 1 : 11,900 : : 30 inches : 357,000 inches - 5-C3 miles. 

 This, therefore, would be the height of an atmosphere 

 pressing with the same weight as our* does, and of which 

 the density is uniformly the same as the air at the 

 surface of the earth. If the specific gravity of the air at 

 the surface to that of mercury, hen the barometer stands 

 at 30 inches, be taken as 1 to 12,000, the height of the 

 homogeneous atmosphere will be 12,000 X 30 inches 

 10,000 yards 6$ miles nearly. Hunce the height 

 of the equiponderant homogeneous atmosphere would be 

 .ii ." u,i;.-. 



I I i< >N'. The weight of the atmosphere, like 

 all the other forces spontaneously offered to us by nature, 

 has, by the ingenuity "I man, been made subservient to _ 



Fif. 



.ints and -,- of ways. 



uleed, the 



weight of the air is only a particular manifest., 

 atmospheric pressure is perhaps the most important f 

 terrestrial phenomena ; and it is not easy to estim i<o 

 the amount of extra toil and privation to which mankind 

 would be subjected, if the air we breathe, like the li^Ut 

 we see, had no appreciable weight. It is our business 

 now to give a short account of some of the contrivances 

 by which atmospheric pressure has lyen turned to ad- 

 vantage in the practical affairs of life, and to explain the 

 principles upon which these contrivances accomplish the 

 purposes intruded by them. The least complicated of 

 these is the Syphon. 



This instrument is simply a bent tube ABC (Fig. 193), 

 employed for the purpose of exhausting a vessel of the 

 liquid it may contain, or of trans- 

 ferring it to another vessel without 

 the practical inconvenience often 

 the practical impossibility of ac- 

 tually pouring the liquid from the. 

 oiii- vessel into the other. 



There are two ways of bringing 

 the instrument into operation : 

 the end of the shorter leg A may 

 be inserted in the liquid, and then 

 the air in the tube withdrawn by 

 the mouth through a small pipu 

 communicating with the tube near 

 the extremity C of the longer leg. 

 In this case there must be a stop- 

 cock between the pipe and C to 

 cut off communication with tins 

 atmosphere pressing tlirough C. 



called the distiller's syphon : it is exhibited in operation 

 in Fig. 194 In the second way the tube is inverted aud 

 filled with the fluid, and the ends A, C closed ; the 

 shorter end is then immersed, and both ends are 

 opened 



Fig. 1M. 



Such a syphon is 



Puppose, as in the first way, that the tul>e, with t'io 

 end A (Fig. 193) iu the liquid and the end C stopped, 

 has been exhausted of air : the pressure of the atmo- 

 sphere on the exposed surface of the fluid iu the vessel 

 acts upwards at A, and forces the fluid into the vacant 

 space with an eneruy sufficient, if it were water, and the 

 tube were straight, to carry it to the height of thirty- 

 two or thirty- three feet ; and to a greater heiijit if it 

 were a fluid lighter than water. As, however, the tube 

 bends at a moderate height B, the ascending column is 

 forced to accommodate itaelf to the course of the tube, 

 and dcxcfiidx into the leg B D C. Arrived at 1!, tho 

 highest point, it descent down B C is expedited by the 

 direct inllnence of gravity upon it ; and C being opened, 

 tin- lii|iiid Hows nut. 



Now it must be observed, that when tho column has 

 attained the height B, it is not forced forward by tint 

 whole of tho atmospheric pressure at E, but only by that 

 pressure diminished by the weight of the column K 1 ' , 

 so that when tin- column had e.\i< n.l. >1 it.sulf to D, if the 

 atmospheric air were admitted, thn upward pressure 

 D, Like that on E, would be equal to the whole atmo- 



