of Edinburgh, Session 1883-84. 
661 
4. Thermometer Screens. By Mr John Aitken. (Plate VI.) 
Part I. 
In meteorological observations the temperature of the air is of 
the first importance, and it is a subject to which a great deal of 
attention is given, thousands of temperature observations being 
made and recorded every day. It is therefore desirable that these 
observations shall be as correct as possible. At first sight nothing 
seems more simple than to take the temperature of the air. All 
that appears necessary is to hang up a correct thermometer of any 
construction, anywhere out of the sun, and the thermometer will 
then indicate the temperature of the air where the instrument is 
placed. We shall presently see that this is very far from being the 
case ; and not only so, but we shall find that it is difficult to get 
two thermometers which will give the same readings, when hung 
near each other in the open air, even though they agree perfectly 
with each other when placed in water. 
It has been the custom of most observers to place the thermo- 
meters in some kind of screen, to protect them from the sun and 
the rain. Many forms of screens have been devised, but Steven- 
son’s is the one that has met with the most general approval. 
This screen consists of a square-shaped box, the sides of which are 
made of double-louvre boards. In this box the thermometers are 
protected from radiation, while the air can circulate freely through 
it. ilo doubt Mr Stevenson’s screen is admirably adapted to our 
climate, where we scarcely ever have calm weather, and the tem- 
perature of the air inside the screen is generally nearly the same as 
that of the air outside it. My attention has, however, been lately 
directed to certain conditions of climate under which this screen is 
not suitable. In France and Italy there is frequently a succes- 
sion of perfectly calm days, in which the sun shines with great 
brilliancy and strength. Under these circumstances I have seen 
the Stevenson screen baking in the sunshine, and the thermometers 
recording temperatures much higher than that of the air outside 
them. These calm, sunny days are not unknown in this country, 
though they may occur seldom ; and it seems worth while con- 
sidering whether something ought not to be done to prevent the 
