156 REPORT—1885. 
Second Report of the Comnuttee, consisting of Professor SCHUSTER 
(Secretary), Professor BALFoUR STEWART, Professor STOKES, Mr. 
G. JOHNSTONE Sroney, Professor Sir H. E. Roscor, Captain 
ABNEY, and Mr. G. J. Symons, appointed for the purpose of 
considering the best methods of recording the direct intensity of 
Solar Radiation. 
Tue Committee, working on the lines of their last report, have given their 
attention to the best form of a self-recording actinometer, and have come 
to the following conclusions :— 
1. It seems desirable to construct an instrument which would be a 
modification of Professor Stewart’s actinometer adapted for self-registra- 
tion—the quantity to be observed being, not the rise of temperature of 
the inclosed thermometer after exposure for a given time, but the excess 
of its temperature when continuously exposed over the temperature of the 
envelope. 
2. As the grant to the Committee will not admit of the purchase of 
a heliostat, it will no doubt be possible to procure the loan of such an 
instrument, and, by making by its means sufficiently numerous com- 
parisons of the instrument proposed by the Committee with an ordinary 
actinometer, to find whether the arrangement suggested by the Committee 
is likely to succeed in practice. The Committee would therefore confine 
their action for the present to the carrying out of such a series of 
comparisons. 
3. The size of the instrument might be the same as that of Professor 
Stewart’s actinometer. 
4. The instrument should have a thick metallic enclosure, as in the 
actinometer above mentioned, and in this enclosure there should be 
inserted a thermometer to record its temperature. Great pains should 
therefore be taken to construct this enclosure so that its temperature shall 
be the same throughout. 
5. The interior thermometer should be so constructed as to be readily 
susceptible of solar influences. It is proposed to make it of dark glass, 
of such kind as to be a good absorber, and to give it a flattened surface 
in the direction perpendicular to the light from the hole. 
6. It seems desirable to concentrate the sun’s light by means of a 
lens upon the interior thermometer, as in the ordinary instrument. For 
if there were no lens the hole would require to be large, and it would be 
more difficult to prevent the heat from the sky around the sun from 
interfering with the determination. Again, with a lens there would be 
great facility in adjusting the amount of heat to be received by employing — 
a set of diaphragms. There are thus considerable advantages in a lens, 
and there does not appear to be any objection toits use. 
The Committee have not drawn their grant (201.). They suggest 
that they be reappointed, and that the unexpended sum of 201. be again 
placed at their. disposal. 
