8 
POPULAR SCIENCE REVIEW. 
instead of a mirror, and thereby preventing the escape of the 
pile’s heat towards the sky. The lens in question was of the 
polyzonal form, having a diameter of thirty inches and a focal 
length of forty-one inches ; giving an image of the full moon 
038 inch in diameter, and thus securing a concentration of 
its light and heat to the extent of 6,000 times. A high full 
moon in December 1834 furnished good conditions for a trial: 
the pile was mounted in the focus of the lens, and the moon’s 
rays were alternately thrown upon and screened from it some 
twenty times during the hour and a quarter that the experi- 
ments lasted. There was an occasional movement of the needle 
amounting to not more than a quarter of a degree, but, as 
Forbes pointed out, the greater part of this was due to the 
dynamical effect of an instantaneous impulse. To form an esti- 
mate of the amount of heat which this deflection represented, 
the number of degrees upon the galvanometer scale equivalent 
to a centigrade degree was ascertained by exposing the pile and 
a thermometer to one and the same source of heat. Two ther- 
mometers were, in fact, used for this comparison ; and the mean 
value of a degree’s deflection of the needle was found to be one- 
fiftieth of a centigrade degree. Supposing that the statical 
effect of the moon-heat upon the pile-needle amounted to one- 
eighth of a degree, we have evidence of heat to the extent of 
T -L_th of a degree centigrade. But this was the measure of the 
condensed heat : assuming that the lens, in consequence of dis- 
persion, reflection, and absorption, concentrated the light 3,000 
instead of 6,000 times, and making a further allowance for the 
proportion of the pile-face covered by the moon’s image, Forbes 
concluded it as improbable that the direct light of the moon 
would raise a thermometer one-three-hundred-thousandth part, 
or, decimally written, 0*0000033 of a centigrade degree.* 
Judging from later results, it seems probable that the gal- 
vanometer needle employed by Forbes was not sufficiently 
delicate to answer to the action of the current furnished by 
the pile. 
Melloni’s new experiments were made in 1846 with a poly- 
zonal lens of a m&tre diameter, constructed for the Meteorolo- 
gical Observatory at Vesuvius. At first he was plagued with 
the abstraction of heat from his pile, as he had been before 
when using a reflector ; but he got over his difficulty by mount- 
ing the lens and pile within doors, allowing the moonlight to 
enter through an open window. When all was in readiness for 
observation, and the rays were first cast upon the instrument, 
the needle moved several degrees in the direction indicating 
heat ; but upon covering and re-exposing the face of the pile, 
• “Transactions of Edinburgh Roy. Soc.,” xiii. 139. 
