266 



Hence the author affirms that Dr. Robison's conclusions 

 must henceforth receive a hmitation. 



Having shown that a positive advantage is obtained by 

 the use of the circular conduit, amounting to about eleven 

 per cent, of the total power, and that this value increases 

 with an increase in the velocity of the wheel up to six feet 

 per second, or more in large wheels, the author contends, 

 that it is practicable to increase the efficiency of the best 

 overshot wheels as now usually made, at least ten per cent, by 

 this application. The only objections ever urged against the 

 conduit were of a merely practical character, and the author 

 shows that improved workmanship, and the modern use of 

 cast iron, of which the conduit may be constructed, and 

 provided with adjustments, render these no longer tenable. 



Drawings of the apparatus used in these researches, and 

 the tabulated results, were exhibited to the Academy. 



Professor Lloyd read a paper " on the Phenomena of Thin 

 Plates in Polarized Light." 



The author stated, that his attention had been drawn to 

 this subject by a letter which he had received from Sir David 

 Brewster, describing a large class of hitherto unobserved 

 phenomena. Sir David Brewster having expressed his de- 

 sire, in this letter, to know whether the wave-theory could 

 furnish an explanation of the facts which he had observed, 

 Professor Lloyd was thus led to undertake the investigation 

 which formed the subject of the present communication.* 



Mr. Airy had long since inferred, from a consideration of 

 the form of Fresnel's expression for the intensity of reflected 

 light, that when light, polarized perpendicularly to the plane 



* The present paper was read in the Mathematical Section of the British 

 Association, last year; and a summary of the results was published in the AUie- 

 nceum, of August, 1S41. The author deferred submitting it to the Academy, in the 

 hope of being able to add an experimental confirmation of some of the conclusions 

 not noticed by Sir D. Brewster. He has, however, been compelled, by the pressure 

 of other duties, to postpone still further this branch of the investigation. 



