200 
Proceedings of the Royal Society 
heated surface, and the circulation is sufficiently rapid to keep the 
walls of the tube at the temperature of the air, and to absorb any 
heat the thermometers may receive by radiation, without being so 
strong as to generate heat by friction. 
The readings of this apparatus have been compared with those 
given by the ordinary Stevenson screen, and an improved form, 
having a double top, and the bottom closed with louvre-boards of 
the ordinary pattern. Besides these, the readings of thermometers 
without screens, simply protected from direct sunshine, have been 
taken. One of these has its bulb enclosed in a silver thimble, 
which reflects diffused radiations of all kinds. Under another 
shade two ordinary thermometers are placed side by side — the bulb 
of one of them having part of its surface blackened. The blackened 
bulb, of course, absorbs part of the heat radiated to it, and the 
arrangement is so constructed that the excess of the reading of the 
blackened bulb above that of the clean bulb, subtracted from the 
reading of the clean bulb, gives the true air-temperature. 
The curves shown are drawn from hourly observations taken during 
twelve hours daily. Since the commencement of these observations 
the weather has not been of the character required to put the various 
screens to a severe test, there having been always more or less wind. 
The actual differences of temperature given, are therefore not nearly 
so great as those observed by Mr Aitken under more favourable 
conditions last summer ; but the differences that do exist are of 
considerable interest. 
The positions of the curves relatively to one another evidently 
depend to a very great extent on the state of the atmosphere with 
regard to cloud, and the influence of the clouds is regulated, not so 
much by their amount as by their position — the effect of a small 
amount of cumulus cloud situated directly above the instruments 
being very marked. 
In the forenoon, as the temperature rises towards the daily maxi- 
mum, the curves for the two Stevenson boxes are steeper than that 
for the fan apparatus ; starting sometimes from a point below it, 
and after crossing it, gradually increasing their distance. This is 
obviously due to the gradual heating of the boxes, the effect being 
much more marked in strong sunshine than in dull cloudy weather. 
If the prevailing conditions persist during a maximum or minimum, 
