122 THE WORLD. 



of twilight, SAB, expires whenever the arc DC, is equal to 9, 



or one-fortieth of the whole circle. The angle B C O is evident- 

 ly a right angle, or 90, consequently, from the well known prin- 

 ciple that-the square of the longest side of any right angled tri- 

 angle, as B O C, is equal to the sum of the squares of the other 

 two sides; the square of B O, is equal to the square of O C, ad- 

 ded to the square of B C. Now B O is the height of the atmos- 

 phere, added to the half diameter of the earth, and O C is also 

 the semi-diameter of the earth, or 4000 miles nearly, whilst B C, 

 which may be assumed equal to D C, is the one-fortieth of the 

 circumference of the earth, or 600 miles nearly. Hence the square 

 of B O is equal to 600 2 , plus 4000*, which is 16,360,000, and the 

 square root of this is 4045, which is the length of B O; subtracting 

 D O, or 4000 miles, leaves 45 miles for B D, the height of the 

 atmosphere. It is probable the atmosphere extends beyond this, 

 as, unless the sky be overcast, there is total darkness in no climate, 

 even at midnight, and therefore the atmosphere must extend to 

 such a distance as to receive the most dilute glimmer after the 

 sun has attained his greatest obliquity, and sunk 90 below the 

 horizon ; this would require ,an elevation of at least 1640 miles, 

 and before the centrifugal force would balance the attraction of 

 gravitation, it is possible it might extend 22,000 miles, and yet, 

 this is scarcely a twentieth part of the distance of the moon. If 

 the atmosphere really spreads out, even to the first mentioned 

 limit, it must, in its remote verge, attain a degree of tenuity which 

 . it would baffle the imagination to conceive. 



It was soon conceived, after the discovery of the pressure of 

 the air, that the height of the mercurial column would vary with 



