On the Heat in the Sun’s Rays. 377 
Art. XXX.—On the Heat in the Sun’s Rays ; by ExisHa Foor, 
(Read before the Amer. Association for the Advancement of Science, Aug. 23, 1856.) 
Tae experiments here detailed were bapa, for the pur- 
pose of investigating the heat in the Sun’s rays. 
Two instruments have been used for the purpose. One 
Leslie's differential thermometer. ‘gga bulbs ‘of ty were black 
ened by holding them in the smoke of burning pitch. When 
Beberally however, I have found it more convenient to use two 
mercurial thermometers, and note their difference. Two small 
and very delicate instruments were procured as nearly alike as 
possible. The stems of both were attached to the same plate 
about two inches apart, and the scales were marked upon it in 
juxtaposition, so that the eye could see the indications of both 
at the same time. Both bulbs were blackened as in the other 
in the same m 
a: in the sun and in the shade were noted, and their difference 
was taken as equivalent to the indications of the. differential 
thermometer. 
The question that first arises is, does the difference between 
the shaded and exposed bulbs afford a correct measure of the 
heat in the a srays? To = Eos I would ask attention be- 
fore proceeding to the experim 
The theory of the diferent eas was accurately in- 
vestigated by Leslie. In one of the foci of two parabolic re- 
flectors he placed a tin ect which was pepe ed toe rs4 
in liquids of different temperatures or frigo 
eoet the heat was ha on one of the bulbs of P his 
oa thermometer: and under all circumstances, the in- 
ions of the instrument were found to be accurately propor- 
to the differences between se temperatures of the canis- 
ter and those of the surroundi 
T have corey eee experiments a8 keeping the canister at the 
uniform heat of boiling water in different temperatures of the 
air, and by suttaing other sources of heat, and have always 
found the results to accord with those Auenes by the distin- 
the he dierent thermometer receives heat from the canister, it at 
‘sons SERIES, VOL, XXII, NO. 66,—NOV., 1856, 
48 
