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1887.] Mr John Aitken on Thermometer Screens. 
the small vertical sunshades 00 were added to the screen. These 
pieces of wood are fixed in a sloping position, to prevent any air 
heated on them from passing downwards towards the thermometer. 
As these pieces will slightly interfere with the air circulation, it 
is possible the screen will act better without them, if observations 
do not require to be made when the sun is very low. They are 
not, of course, required on the north side of the screen, or where 
houses or trees shut out a low sun. The best size for the sunshade 
has yet to he determined. If made larger than shown, say 3 feet 
square, then only a very low sun could have any effect on the read- 
ings. This screen for future reference is called screen C. 
Let us look at the action of this screen when placed in the open 
air. It will he seen that the air has a very free circulation through 
it ; the plates being horizontal and placed at a distance from each 
other, the air has a perfectly free passage through it from whatever 
direction it may blow. It will he further noticed that the bulb is 
perfectly protected from radiation from all bodies outside. Turning 
now to the manner in which the heat absorbed by the screening 
surfaces is prevented from affecting the readings of the thermometer : 
First, the large sunshade AA prevents any part of the screen proper 
from being heated by the direct rays of the sun, and it has thus only 
the diffused radiation to contend with. The under side of AA will 
be a good deal heated, hut the hot air in contact with it will pass 
between AA and C, and not come into contact with the bulb. 
The plate E, with the air space between it and D, prevents the heat 
radiated from the ground passing upwards to the upper surface of 
D. If D was a perfect non-conductor, E would be unnecessary. 
The only hot air that really gets into the screen surrounding the 
bulb is the air heated on the under surface of C and upper surface 
of D, and also that heated on the upper and under surfaces of F. 
As will be seen from the construction of the screen, very little heat 
falls on the surfaces of C and D, so they will be but little heated ; 
but the air that gets heated on these surfaces does not come into 
contact with the bulb, but tends to flow straight through and out at 
the other side of the screen, keeping to the surfaces of the plates. 
This will not be the case when the wind is strong, but when the 
conditions are trying, that is, with little wind, it seems probable 
that the heated air will pass through without mixing much with the 
