680 
Proceedings of the Boyal Society 
wire clotli, for instance, freely exposed to the air, will get but little 
heated even in sunshine, and will neither radiate nor reflect much heat. 
Attempts have been made to improve the working of the screen 
(fig. 3) by cutting off all radiation from the ground. First, a small 
non-conducting screen was fixed horizontally under the tube, but 
leaving plenty of space for the air to enter freely ; but no decided 
advantage was obtained. Then a small screen was placed in the 
tube, just under the thermometer bulb, to check the radiant heat 
falling on it, but no reduction in temperature was effected. No 
great advantage can be got from placing a small screen under the 
bulb. If the effect of the upward radiation from the ground is to 
be entirely checked, it must be cut off from the inside surface of 
the tube, as well as from the bulb ; as I find that, to get rid of the 
last fraction of a degree above the temperature of the air, it is of 
far more importance to check radiation, than to increase the current 
past the bulb ; as a very small amount of radiation is equal to a 
strong current of air, and the changes of temperature in a thermo- 
meter are far more quickly accomplished by radiation than by con- 
tact with the air. These considerations show us that the tube 
surrounding the bulb, in addition to being a non-conductor, ought to 
be made of some substance that has small capacity for heat, so that 
its temperature may change quickly with that of the air ; because 
the changes in the thermometer are caused more by the exchange 
of heat with the surface of the tube than by contact with the air 
passing over it. In these instruments the inner concentric tubes 
are made of blotting paper kept in shape by thin metal tubes. 
Hair felt, if it could be got of suitable shape, might be better than 
paper. 
The screen constructed according to the plan shown in fig. 2 
does not give quite such good results as the one made as shown 
in fig. 3. In the former screen the draught tube is of a less 
diameter, and the lower part is less protected by non-conducting 
coats and air passages than the latter. This suggests that the 
fraction of a degree too high, given by the thermometer in screen 
fig. 3, is partly caused by heat conducted through the lower 
end of the tube, and radiated across the air passages. This sup- 
position has been confirmed by an examination of the air passages 
by means of a thermometer. The temperature of passage next the 
