674 
Proceedings of the Royal Society 
have an important influence. Anything which prevents the free 
circulation of the air near a bulb, or which comes in contact 
with the air before it reaches the thermometer, will cause it 
to read too high. The system of sinking half of the bulb 
into a wooden frame cannot but interfere with the correctness of 
the indications, save when the thermometer is perfectly protected 
from radiation, and even then it makes the instrument sluggish. 
All thermometers not protected by proper screens, such as those 
used in gardens, and for other rough purposes, ought to have 
the bulb perfectly exposed to the air, and as far away from all 
surfaces as possible. 
To illustrate the effect of size and radiation, I may mention that 
when I found that small bodies were cooler and nearer the tempera- 
ture of the air than large ones, I repeated my experiment with the 
horizontal shade and vertical draught tube, but in place of enclosing 
the thermometer bulb in a three-inch tube, a very small one was 
employed, just large enough to allow a current of air to flow freely 
between it and the bulb. The result, however, was that the upward 
current did not quite compensate for the larger size of even this small 
tube, and the thermometer was never quite so low in the tube as 
out of it, and exposed to the wind blowing at the time. No doubt, 
in a calm day this arrangement would have prevented any great 
rise in the temperature, and the arrangement would have been better 
than some screens or than no screen. 
These experiments, while they indicate certain errors in the 
construction of thermometers, suggest some possible advances and 
improvements. We have seen that the temperature of a body 
exposed to radiation in the open air is colder the smaller it is. The 
meaning of which is that the smaller a body is, the less effect radiant 
heat has on it, and the nearer its temperature is to that of the air. 
Carrying out this line of thought, we see that if we could construct 
thermometers sufficiently small in the bulb, they would indicate 
nearly the true temperature of the air, and if small enough they would 
indicate nearly the true temperature even in the sunshine. A ther- 
mometer made of a fine wire, for instance, would be well enough 
protected if simply screened from the direct rays of the sun. At 
present I am attempting to get a mercury thermometer made with a 
very fine bulb, which it is hoped will give nearly the true tempera- 
