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Proceedings of the Boyal Society 
necessary condition is that they are fixed, and that the air moves 
over them, whereas the dust particles move with the air. But 
may not a precisely similar cooling effect he brought about in 
another way? In the case of a very small object, like a dust 
particle, a molecule of air, which has touched it and got heated, 
passes into the widening space outside, and scarcely never returns to 
the dust particle; so that every air molecule that touches the 
particle is cold, and the heat received by the particle is not con- 
served by the heated molecules keeping near it and protecting it 
from contact with the cold ones. In this way the molecular 
movements accomplish for the dust what the mass movements do 
for fixed bodies. 
These experiments also suggest some thoughts regarding the 
thermal experiences of different forms of life. The tiny insect, 
as it sports in the sunshine, is the one above all others which is 
generally supposed to have the greatest enjoyment in the warmth of 
the sun’s rays. From what we have seen, we now however know, 
that it is the one above all others which has the least reason for 
gratitude, as it can scarcely receive any heat from the sun. Let the 
sun shine as brightly as it may, it can scarcely warm to a perceptible 
degree the tiny limbs of these fluttering insects ; and if it will enjoy 
the sun’s heat, it must descend to earth. And what a change does 
it meet with there when it lights on branch or stem of tree ! In 
a moment it passes into an atmosphere twenty or thirty degrees 
w^armer than the sunlit air in which it delights. On the other 
hand, this immunity from the heating effect of the sun makes life 
possible to these insects under a sunshine that drives the larger 
animals to seek the shade. 
.Part II. 
(Added 23rd August 1884.) 
Since the first part of this paper was written, there has been some 
weather suitable for testing the efficiency of the thermometer screens 
under trying conditions. From the beginning of the month till the 
22nd there were a number of warm, bright, and nearly calm days. 
Under these conditions it is well known that it is difficult to get 
the true temperature of the air ; advantage was therefore taken of 
