of EcUnlmrgh, Session 1883-84. 
675 
ture of the air when placed under a sunshade such as that shown in 
fig. 4, hut without the louvre box. I regret that the time taken in 
getting a satisfactory instrument of this kind made prevents me 
from being able to say how near the indications of such an instru- 
ment will approach the true temperature of the air. 
These experiments have also suggested the construction of a 
cheap and simple radiation thermometer, which has been made in 
the following way : — An ordinary thermometer, or a registering one 
by preference, is let into the surface of a thick piece of wood. The 
best size of this casing has not been determined, but it ought to he at 
least a foot square, or, better still, large enough to encase the whole 
stem of the thermometer. The thermometer is let in to such a 
depth that the bulb is level with the surface of the wood. Cement 
is then put in to fill up all inequalities and restore perfect smooth- 
ness to the board. The surface is afterwards blackened. It may 
be mentioned that, in order to make the instrument act quickly, a 
large cavity is cut in the wood, into which the bulb of the 
thermometer is placed, and the cavity is packed with cotton-wool, 
and covered over with a thin layer of the cement. There have 
only been two opportunities for making observations with this 
instrument. On one afternoon it was placed in sunshine, and 
alongside was hung a thermometer with a blackened bulb, but 
exposed to the air. The blackened bulb only rose to a tempera- 
ture of 78°, while the one fitted into surface of the wood rose to 
105°. 
This effect of the size of the surface of a body in determining the 
amount of the cooling effect of the air will, I hope, help to explain 
some difficulties. And one cannot help asking here, whether we 
have not in this a suggestion as to the explanation of why it is that 
dust in the sun’s rays, focussed by a powerful lens, escapes being 
burned. It does seem strange to any one accustomed to use a “ burn- 
ing glass ” to light cigarette or pipe, that the organic dust passes 
through the focus of his lens without being affected; but the 
moment the focus falls on the solid tobacco, it at once responds 
with a cloud of smoke, blow why is this '? Does not the difficulty 
of heating small bodies by radiant heat suggest the answer? It 
is true the conditions are not alike. The small bodies we have 
been experimenting with are comparatively large ; and further, a 
