] .|.]j SPECIFIC CR.WITY OF M(.I1T 



tain mechanl- momentum upon a balance, but the transmission through 



fa y * different glasses will vary, as the lenses may happen either to 



differ in density or transparency ; and consequently will give 

 different results. You also advanced (Introduction to Na- 

 tural Philosophy) some ingenious arguments to decide ife 

 amazing subtlety, founded upon undoubted principles. 



Mav perhaps ^^g following analogy will appear perhaps hypothetical ; 



analogically, however, such as it is I will submit it to the candid and dis- 

 criminating', to determine whether the conclusions are sub- 

 stantial or premature. 



Undulations of The resistance of fluids is as their densities reciprocally; 



their gravities, therefore it may be presumed, that the undulations of diffe- 

 rent mediums will, bear. the baine proportion to one another, 

 as their specific gravities. 



Velocity of It has been demonstrated by the accurate observations and 



those of light, ^coyeries f £>r. Bradley on the aberration of light, that 

 this medium is conveyed from the sun to the earth, or in 

 other words, an undulation of light reaches the earth from 

 the sun, in the space of 8' 7*5" of time. Taking the mean 

 apparent diameter of the sun at 32' 1*5 ''» and his mean ho- 

 rizontal parallax at 8'72", as determined by Dr. Maskelyne, 

 and the semidiameter of the earth at 3964 miles, we shall 

 find the sun's real diameter to be 873,489 miles, and his dis- 

 tance from the earth equal to 93,334,047 miles: therefore 



the velocity of lieht will be determined thus, J ~ >J 1 '■ ~ 



J b ' 8' 7-5" 



191,434 miles in one second of time, or 1,010,771,520 

 feet. 

 Undulations of* According to Hales (Statics, Vol. TI, p. 331) the velo- 

 *** ared* 1 * c ' t * v °^ modulating air is to the velocity of undulating water 

 as 8C5 to 1, or as their specific gravities. The motion of 

 sound is found to be 1130 feet in one second (Young's Syl- 

 labus of a Course of Lectures, 1802): then, as the velocity 

 of sound is to the velocity of light, so is the specific gravity 

 Hence Kgfat of air to the specific gravity of light, according to the fol- 



4 B • ;m>s 1010 rT 71520 



lighter than lowing formula ; — : — = 894,583 times lighter than at- 

 mospheric air; or it will require 1553 cubic feet of light to 

 weigh one grain. If we compare them with water, taken as 



unity 



air 



