TRANSACTIONS. 



THE MECHAXTCAL ACTIOX OF RADIATION. 

 By LeRoy C. Cooley, Ph.D. 



[Read before the Albany Institute, February 1, 1876.] 



The motion of light bodies under the influence of radiant 

 heat and light seems to have been noticed, independently, 

 by several observers, at long intervals during the last half 

 century, but only within the half decade just past can it 

 be said to have gained a place among the phenomena of 

 acknowledged interest in science. 



In the Edinburgh New Philosophical Magazine for 1828 is 

 a record of what seems to have been the earliest experi- 

 ments on this subject. They were made by Mr. Mark 

 Watt, and I quote from his interesting paper the following 

 description of the first instance of a light body indicating, 

 by its motion, the impression it received from the sun's 

 rays. '-Twelve or fifteen magnetized sewing needles 

 were stuck into a thin circular slice of cork an inch in di- 

 ameter, at a distance of one-sixth of an inch from each 

 other. The heads of the needles were so fixed into the 

 piece of cork that they stood perpendicularly and all the 

 points, being south poles, stood uppermost. The cork 

 was then placed on the center of a surface of water IJ 

 feet in diameter. The needles, in this situation, beino- 

 prevented from evincing any polar attraction by their per- 



Trans. ix.'\ 1 



