THE HEAT OF THE MOON. 
9 
much to Melloni’s surprise, it turned towards cold. He traced 
the cause of this to draughts of cool air impinging upon the 
thermoscope. These could have been remedied by ordinary 
means ; but the more philosophical cure was to cover the 
pile with plates of glass, which would allow heat to pass through 
to it but yet keep all air-tight. Two screens were then inter- 
posed, and upon the next favourable occasion the experiments 
were renewed. This time they were perfectly successful : the 
needle, stationary when the pile was uncovered, soon began to 
move slowly to the heat-side of the scale, and at the end of five 
minutes’ exposure it reached its maximum of deflection, 3*7 
degrees of arc. The pile was covered, and it returned to zero ; 
uncovered, and it turned heat-wise. This alternation was re- 
peated many times, and always with the same qualitative result. 
Subsequently, trials were made in the presence of MM. Belli, 
Mossotti, and Lavagna, and many other distinguished savans, all 
of whom went from the chamber, says Melloni, convinced that 
the light of the moon is calorific.* No detailed observations 
are given by Melloni, but he says that he assured himself that 
the lunar heat varies, as one might suppose it would, with the 
age of the moon and with its height above the horizon : he in- 
tended to determine the actual amount of the heating effect, 
but it does not appear that he ever did so. Prof. Zantedeschi 
confirmed Melloni’s conclusions in 1848 by observations made 
with a mirror of 0*6 metre diameter, and a pile made by Gfour- 
jon of Paris, used upon the clear full moons that shone in the 
summer sky of Venice. 
Favourable circumstances offered for catching some of the 
heat-rays that are ordinarily absorbed by the lower atmo- 
sphere, when Professor Piazzi Smyth, in 1856, ascended Tene- 
riffe to test the supposed advantages of an elevated station 
for delicate astronomical observation. A thermopile accord- 
ingly formed part of the equipment of the expedition, but no 
burning lens or mirror was taken to be used with it. The 
pile was simply furnished w 7 ith a polished metal cone of rather 
larger base than the area of the face, and the experiments 
were made by alternately turning the cone towards the moon 
and to a part of the sky 20 degrees east or west of it. Two 
nights in August 1856, that of the full moon and the following 
one, afforded favourable opportunities for observation, though 
the luminary was rather low, having an altitude of about 
45 degrees. The effects of heat w r ere unmistakable : on each 
night some thirty readings of the galvanometer were taken with 
the pile alternately on and off the moon ; and the mean de- 
flection in the direction indicating warmth derived from all the 
* 11 Comptes rendus/’ xxii. 543. 
