668 
Proceedings of the Boyal Society 
tube a and the fan c are connected by means of the pipe e. When 
the fan is revolved, a rapid current of air is drawn in through 
the tube a, and over the thermometers, which quickly acquire the 
correct temperatures. When the fan is turned very slowly the wet 
bulb falls a little below the dry bulb, and falls further and further 
as the velocity is increased ; but this fall only goes on till a certain 
slow velocity is attained, after which any increase in the rate at which 
the fan can be driven does not alter the temperature, either of the wet 
or of the dry bulb, but only causes them to acquire the correct tempera^ 
tures more quickly. With this apparatus we always get the same 
difference of temperature with the same conditions of atmosphere. It 
will be noticed that the upper part of the fan is cased in to prevent 
the air currents put in motion by it from rising and entering the 
suction pipe a, and coming into contact with the thermometers. 
When this fan arrangement is put into action, the air drawn into 
the thermometers does not come into contact with much heated sur- 
face, and the circulation is sufficiently rapid to keep the sides of the 
air passage at the temperature of the air, and also quick enough to 
absorb any heat the thermometers may receive by radiation ; it 
may therefore be assumed that the readings given by the dry bulb 
show very nearly the true temperature of the air. This apparatus 
has accordingly been used in preference to the Froude thermometer 
for testing the action of the different forms of thermometer screens, 
their degree of perfection being judged by the nearness with which 
the readings of the thermometer placed in them agreed with those 
given by this fan arrangement. The readings were also occasionally 
checked by means of the sling arrangement; which, however, is 
very difficult to use, especially when the sun is shining. 
On the Temperature of different sized Bodies. 
Before concluding this paper, I wish to record the results of some 
observations made to test the effect of size on the temperature of 
bodies placed in the open air, as these results have a direct bearing- 
on our present subject. 
Let us first consider the temperature of bodies under the most 
arrangement has been found much more convenient than the handle, as an 
impulse can he easily given by means of it to the fan while it is in rapid 
motion. 
