662 P'roceedings of the Royal Society 
too high readings given on exceptional days in the screens at 
present in use. 
To get the true temperature of the air, the simplest plan seems to 
he to place the thermometer in some sort of box or tube where it 
will be protected from radiation, and to cause a rapid current of 
the air to be tested to rush over it. The only difficulty in prac- 
tically carrying out this method for general meteorological purposes 
is to get some simple means of keeping up a constant and rapid 
current of air. There are many ways in which this may be done, 
but none of them are very satisfactory. We might, for instance, 
by means of a pipe, connect the box containing the thermometer 
with a chimney, under which a fire was constantly burning ; or 
we might keep up the draught by means of a jet of gas burning 
in a vertical tube placed over the thermometer box ; or we might 
use a revolving fan and water power, or a water jet, or any 
other easily applied method of causing a current of air at the 
place where the observations are to be made. All these plans, 
however, require special apparatus and constant attention to see 
they are in working order ; and though some of them might be 
used in certain conditions, yet none of them are suitable for general 
adoption. A simpler and self-acting arrangement has therefore 
been designed, by means of which a current of air is kept con- 
stantly flowing over the thermometer. This apparatus consists 
of a screen and a draught tube combined, the draught tube 
being heated and put into action by the sun’s rays. The screen 
acts during ordinary dull weather when the wind is blowing, and 
the draught tube keeps up the air current over the thermometer 
when the wind falls. This draught tube is always in action when 
required, and most when most required, always acting more power- 
fully the stronger the sunshine. 
Plate VI. fig. 1 shows the theoretical form of a thermometer screen 
made on this principle. The thermometer t is placed in the lower end 
of the vertical tube ah. The lower part a of the tube is constructed 
of some non-conductor, or some other arrangement is made to prevent 
radiant heat getting to the thermometer. The upper part h of the 
tube is made of metal, and painted black. Suppose this tube placed 
vertically anywhere in sunshine, the upper part of the tube will 
become heated, and will warm the air inside. A rapid current will 
