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 11 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 
