'Elements of Astronomy. 13 



latitudes, must be shortened at the equator in order to ren- 

 der the oscillations isochronous. [A pendulum 39* 197 

 inches long will swing seconds at the poles : but, in order 

 that it may swing seconds at the equator, it must be re- 

 duced to 39-027 inches.] 



The centrifugal force at the equator is nearly Trla-th of 

 gravily. It the rotation of the Earth were 17 limes more 

 rapid, the centrifugal force would be equul to that of gra- 

 vity : and bodies at the equator would not have any weight. 

 A rare and elastic fluid surrounds the Earth, which is 

 called the atmosphere. Neither the temperature nor density 

 of this fluid is uniform; but diminishes in proportion to 

 its diitance from the surface of the Earth, and is also af- 

 fected by other circumstances. 



If the density of the atmosphere were every where the 

 same and its temperature at zero, and the height of the ba- 

 rometer at 29-9!2196 inches, the height of the atmosphere 

 would be 26067 ftel; or 3*7 miles. 



The atmosphere is a heterogeneous substance. Out of 

 100 parts, 79 are azotic gas, and the remaining 21 are oxy- 

 cren gas. I'his is found to be universally the c.i«e, in what- 

 ever season or whatever climate the experiment be tried. 

 This proportion is also found to exist in the highest points 

 of the atmosphere that have been reached by means of 

 balloons. 



A bodv projected horizontally to the distance of about 

 4*33 miles, if there were no resistance in the atmosphere, 

 would not fall again to the surface of the Earth, but would 

 revolve round il^as a satellite; the centrifugal force being 

 then equal to its gravity. 



The rays of light do not move in a straight line through 

 the atmosphere; but are inllecicd continually towards the 

 Earth : so that the stars appear more elevaie'd on the hori- 

 zon than they really are. 



We find, from the most. accurate observations, that the 

 refraction, which the atmosphere produces, is independent 

 of its temperature, and proportional to its density. But, 

 as the density varies according to the temperature; it is 

 neccssarv to attend not only to th<5 slate of the barometer, 

 but also of the thermometer. 



The humidity of the air produces very little effect on its 

 refractive powers, and may therefore be safely onntled. 



The temperature of the uhole atmosphere being sup- 

 posed at zero, its density will diminish in a geometrical 

 progression, accordmg to its distance from the surface of 

 ihe Ejirth : and we find by experience that, the height of 



th« 



