56 Proceedings of Royal Society of Edinburgh, [jan. 17 , 
the air. When the sun is at its highest the air is receiving its 
maximum heating effect, but owing to its inertia it does not acquire 
its maximum temperature till a later hour, till near two o’clock, at 
which hour the amount of heat received is balanced by that lost. 
After that the temperature of the air falls, but the curve represent- 
ing its fall is later than A, owing to the inertia of the air, which 
affects a falling as well as a rising temperature. The same ex- 
planation applies to the yearly maximum, and shows why it does 
not occur in J une, when the sun is highest and the greatest number 
of hours daily above the horizon, but at a later date, when its heat- 
ing power has considerably diminished. 
The curve C in the figure represents the effect which a still 
greater inertia has on the rise of temperature in a body heated 
by the air. As the temperature of A never falls quite to that of 
C, the curve C never falls, but only varies in the rate at which it 
rises. It will be as well to note here, that all these effects of 
inertia in checking and retarding the heating of large bodies are 
quite apart from the question of radiation and its effects on large 
and small bodies, which, as has been shown in a previous paper, 
acts in exactly the opposite way, and tends to heat large bodies 
to a higher temperature than small ones. 
Let us now turn to the practical consideration of the subject, and 
see what the effect of inertia really is on the readings given by 
different arrangements of apparatus. On PI. II. fig. 2, are shown 
curves of temperatures drawn from readings given by different 
arrangements, each having a different inertia. The curve FB shows 
the readings of a very fine bulbed thermometer. This thermometer, 
as was explained in a previous part of this investigation, was con- 
structed to be used as a standard of air temperatures, with which 
to compare the readings given by the different screens. The bulb of 
this instrument is 25 mm. long, but it has a diameter of only about 
1*5 mm. When in use, it is exposed under a horizontal sunshade 
in the manner described in Part II., and the bulb is protected from 
radiation by means of a sheath of pure silver, which fits it closely, 
but does not press upon it. 
The readings given by this instrument were considered to be 
nearer the true temperature of the air than those given by any 
other arrangement, as they always kept lowest while there was any 
