and Polarization of Heat. 137 



mensions of the intercepted cylinder of rays, we shall find the 

 concentration to exceed 6000 times. But even if we admit that 

 half the rays are reflected, dispersed and absorbed, we shall have 

 still an effective increase of 3000 times. 



12. My experiments were made on the 16th December 1834, 

 between 9 and 1 1 o'clock, the moon being only 18 hours past full, 

 and (towards the close) less than 2 hours from the meridian. She 

 was also remarkably high, having a declination of 25° north. The 

 thermal pile, which was particularly commodious for the experi- 

 ment, had one extremity elevated to the proper angle, and being 

 placed accurately in the focus of the mirror, the moon's image was 

 brilliantly thrown on the extremity of the pile. The sky was on 

 the whole very pure, though an occasional milkiness was per- 

 ceived, but the best observations were made at the clearest mo- 

 ments, because then the air was also most still ; for though the 

 instrument was placed in a most sheltered spot, the faintest breeze 

 was indicated by the altered temperature inducing a deflection of 

 the needle, and with such promptitude, that I generally could per- 

 ceive in this way its approach before I could feel it. The action 

 of the lens was so perfect, that the image was extremely sharp, and 

 the spots clearly defined. The lunar rays were alternately 

 screened and admitted by an assistant passing a sheet of paste- 

 board across the surface of the lens next the moon ; for when it 

 was interposed between the lens and the instrument, a sensible 

 disturbance took place. By these and other precautions, the 

 needle was steady beyond my expectation, and during an hour 

 and a quarter that the observation lasted, I had probably at least 

 twenty perfectly unexceptionable comparative observations, free 

 from the influence of wind, and which invariably gave not the 

 faintest indication of warmth. When I got a deviation of the 

 needle at the moment of unscreening the moon's rays, I verified 

 it by screening them instantly, and watching for a return to zero, 

 but I was always disappointed. I feel quite confident that the 

 effect, if there was any, could not amount to a quarter of a degree 



VOL. XIII. part i. s 



