THE HEAT OF THE MOON. 
5 
Height and density of clouds are unheeded. Thermometer 
readings, on the other hand, are more definite, and less subject 
to erroneous classification ; and we would rather accept their 
testimony of lunar influence on terrestrial temperatures, indi- 
rect though it be, than anything not susceptible of instru- 
mental measurement. It would be a great point if we could 
measure the moon’s dark heat directly, but this there seems 
little hope of doing, unless, as Professor Smyth suggests, we 
ascend to the level of the high clouds which he saw evaporated 
by the full moon — an altitude of about three miles. 
The visible heat rays, and those which are not intercepted by 
the atmosphere, we can now, thanks to modern means of thermo- 
metry, measure with some pretensions to accuracy. Whether 
the moon’s light possesses perceptible warmth has been a point 
of anxious and tentative inquiry among astronomers and phy- 
sicists for a century and a half at least. The first observer who 
recorded a trial of it was, I believe, Greminiano Montanari, who, 
in 1685, thought he had indications of lunar heat in an ordi- 
nary dilatation thermometer. The second was Tschirnausen, the 
famous burning-glass maker. He published, in the year 1699, an 
account of a wonderful double lens, which concentrated the solar 
rays so far as to make them melt and fuse metals. It was 
formed of a four-foot burning-glass, with another lens of smaller 
size behind it. In telling of a number of its achievements, he 
mentions, little more than incidentally, that he turned it upon 
the full moon, but, though the image formed in the focus was of 
great brilliancy, there was no sensible heat.* He does not say 
what thermometer he used. Six years after, in 1705, La Hire 
the younger made ail experiment with a burning mirror, belong- 
ing to the Paris Observatory, of 35 inches diameter, and an air 
and mercury thermometer of the construction then recently pro- 
posed by Amontons. The bulb of this instrument was two inches 
in diameter, and when it was placed at the focus of the mirror 
the moon’s reflected image just covered it. The height of the 
mercury was noted, and the condensed moonlight was kept 
upon the bulb for a considerable time, but there was no altera- 
tion in the reading. This one trial satisfied La Hire that the 
moon’s light was heatless, f 
We find no record of attempts at lunar thermometry during 
the hundred years following the date of that of La Hire. But 
in 1820 attention was again turned to the subject by Professor 
Howard, of the Maryland University in the United States. 
Thermometers had by this time improved in construction : the 
instrument used by Howard was a modification of the differential 
one proposed by Leslie. The condenser was a mirror thirteen 
* 11 Ilistoire de l’Acad^mie,” 1G99, p. 90. 
t Ibid., 1705, p. 340. 
