140 Experimental Philosophy. [Lecture 10. 



case, or box. The rays of the sun were thrown 

 upon the broad part of the vane, or copper plate, 

 by a burning-glass of two feet diameter, in con- 

 sequence of which it was observed to move re- 

 gularly at the rate of about one inch in a second 

 of time. Upon this experiment a very curious 

 calculation is founded. The instrument or vane 

 weighed about ten grains, and the velocity with 

 which it moved was at the rate of one inch in a 

 second. The quantity of matter therefore con- 

 tained in the rays of light which struck against 

 the vane in that time amounted to about the 

 twelve hundred millionth part of a grain: the 

 velocity of light exceeding the velocity of the 

 instrument in about that proportion. The light 

 in this experiment was collected from a surface 

 of about three square feet, and as it was from a 

 concave mirror *, only half the quantity was re- 

 flected. The quantity of light therefore incident 

 upon a square foot and half of surface is no 

 more than one twelve hundred millionth part of 

 a grain. But the density of the rays of light at 

 the surface of the sun is greater than at the 

 earth, in the proportion of forty-five thousand 

 to one. From one square foot of the sun's sur- 

 face, therefore, there ought to issue, in the space 

 of one second, one forty thousandth part of a 

 grain of light to supply the consumption. More 

 than two grains a day therefore is, according to 



* Mirrors or looking-glasses reflect about half the light 

 that fulls on them perpendicularly. 



