436 Proceedings. 
lished by Dulong and Petit, as a result of their experiments on the rate of 
cooling of various bodies. A rather smart discussion occurred, in which 
many eminent men took part, each of whom appears to have retired from 
the controversy without seeing reason to modify his own opinion, a circum- 
stance which may assure us that we aré yet a long way from a sure ground 
on which to base a definite calculation of the solar temperature. 
Attempts have been made by many physicists to determine the radial 
energy of the sun since the day when Newton first made the calculation by 
which he estimated that the comet of 1680 was subjected at its perihelion 
to a temperature of 1,484,000° Fah. As a preliminary to certain considera- 
tions which have occurred to my mind during the study of this subject, I 
must ask you to allow me to describe briefly the methods which have 
most recently been employed to test the energy of the solar radiation, and 
the deductions which have been drawn from observations thus conducted. 
The method which I have selected for description is that of the thermo- 
heliometer. This instrument, which has been variously fashioned 2 
accordance with the ideas of those who have used it, consists essentially : 
of two concentric cylinders placed within one another. The annular space 
between these is filled with water or oil maintained at a known constant 
temperature. At one point a tube passes through both cylinders by means 
of which a thermometer is introduced into the interior of the smaller one. 
The bulb of this thermometer is blackened, and the rays of the sun are 
allowed to fall upon it through the circular opening at one end of the 
cylinder. The whole apparatus is of course attached to a heliostat, by 
means of which the axis of the cylinders is kept constantly directed to the 
face of the sun. The thermometer thus exposed to the sun’s rays shows @ 
rapid increase of temperature up to a certain point, after which its indica- 
tions vary directly with the increasing or decreasing altitude of the sun, 8° 
long as the sky remains equably clear. From a multitude of observations 
taken at different zenith distances of the sun, itis not difficult to calculate 
what temperature would be indicated by the thermometer if the sun were 
actually overhead. These measures, at various altitudes of the sum above 
the horizon, also give us the means of determining the mean absorption of 
heat by the atmosphere, since the difference between the effective radiation 
at low altitudes and at noon is caused solely by the difference in the depths 
of air which the solar rays traverse. The results obtained by different 
observers with instruments constructed on this principle vary with the 
amount of precaution taken to secure (1), the steadiness of temperature on 
jo Aiea eel (2), freedom from draughts and other causes whi¢ 
“2 € indications of the thermometer; and (8), equable heating of 
whole mass of the bulb of the instrument by the sun’s rays. 
i 
