THE HEAT OF THE MOON. 7 
tion in the science of heat measurement: Seebeck, in 1822, 
discovered the power of thermal currents to excite electricity in 
metallic conductors ; and Nobili adapted the discovery to the 
construction of a thermoscope which permitted the making of 
experiments that would have been declared impossible a very 
short time before. The years about 1830 found the inventor of 
the thermopile, in conjunction with Melloni, engaged upon in- 
vestigations involving the measurement of temperatures sepa- 
rated by very small fractions of a degree upon any existing 
scale. The passage of heat through transparent bodies, the 
bodily warmth of insects, the calorific exhalations accompanying 
the luminous glow of phosphorus — these were the subjects 
upon which were first tested the powers of the new instrument, 
which was so delicate that it felt the warmth of a human 
body thirty feet removed from it. Having refuted the common 
opinion that luminous phosphorus exhibited the phenomenon of 
light without heat, Melloni sought to disprove the similar idea 
sometimes entertained with regard to the light of the moon. 
The pile he employed was formed of thirty-eight pairs of bis- 
muth and antimony bars, soldered together at their alternate 
ends, and packed closely, though electrically isolated, into a 
metallic hoop, the first and last bars being connected by wires 
with the coil of a delicate galvanometer. A conical reflector 
surrounded the exposed ends of the bundle of bars which formed 
the <( face ” of the pile ; this face was coated with lamp-black : 
the lunar rays were concentrated upon it by a concave metallic 
mirror, the diameter of which Melloni does not mention ; but 
whatever effect they might have produced was completely 
shrouded by that of the cold received from the sky, or, to speak 
more correctly, the heat escaping by radiation from the exposed 
surface of the pile.* It must be borne in mind that currents 
are developed in the bundle of plates or bars whenever the 
equilibrium of temperature between the two faces is disturbed 
in any way . If one face is boxed up and the other is opened 
to the view of a clear sky, the latter will cool more rapidly than 
the former, and the effect will be as if the covered face had been 
actually warmed. Melloni, in his lunar experiments, found his 
galvanometer always pushed to its limit of divergence from this 
cause ; and for a time he was frustrated. Several years after, 
adopting precautions to neutralise the frigorific effect of the 
clear sky, he was able to obtain decided heat indications, as we 
shall presently see. In the meantime, Professor Forbes, taking 
advantage of the delicacy and prompt action of a pile he had 
constructed for some experiments upon the refraction and polari- 
sation of heat, renewed Melloni’s experiment, using a lens 
* “ Annales de Cliimie,” xlviii. 211. 
