1887.] Mr John Aitken on Thermometer Screens. 
55 
in which the air is heated. In PI. II. fig. 1, the cufve A is 
supposed to represent the changes in temperature of the air, drawn 
to a scale on which the vertical lines represent half minutes, 
while the horizontal lines represent half degrees. During one 
minute the temperature often rises or falls more than one degree. 
For convenience of illustration, this curve, representing the 
temperature of the air, is shown as a smooth curve. In reality 
it is not likely to he so, but in all probability is a very irregular 
one. Suppose then the curve A represents the temperature of the 
passing air, then the curve representing the temperature of any very 
small body, such as a cobweb, will follow this one very closely. 
But if the body is of any size, then the curve of its temperature will 
be something like the curve B. Its temperature will rise and fall 
with that of the passing air, but the two curves will not rise and fall 
together, because the temperature of the body will go on rising after 
that of the air has begun to fall, and it will continue to rise so long 
as the air is the hotter of the two. In the curves, the temperature 
of B is shown to be rising for more than half a minute after A has 
attained its maximum, and it is not till A has fallen more than half 
a degreee, and has the same temperature as B, that the latter ceases 
to rise, and the curve of its temperature becomes horizontal. After 
this A and B both fall, hut A more quickly than B, and B does not 
attain its lowest point till after A has passed its lowest, and risen 
to a certain amount, and acquired the temperature of B, after which 
both curves rise, but A more quickly than B. 
The points to he noted here are : First, that if the top of the 
curve A had been the maximum for the day, then the inertia of B 
would have prevented it acquiring the maximum temperature, so 
that any arrangement of screen having a large inertia will tend to 
give a lower reading than one with a small inertia. 
The second point is, the effect of the inertia in retarding the time 
of maximum temperature. The curve B does not arrive at its 
maximum till some time later than the curve A. These considera- 
tions help to explain why the daily maximum temperature does 
not occur about mid-day, when the sun is at its highest, but at a 
later hour. If we suppose the curve AA', continued as shown by 
the dotted lines in the figure, to represent the intensity of solar 
radiation, then the curve BE' will represent its heating effect on 
