C65 
of Edinburgh, Session 1883-84. 
between them for the free circulation of air, to keep them cool and 
to prevent the heat absorbed on the top during sunshine penetrating 
to the thermometer. When fitted up the discs are placed hori- 
zontally, that the wind may blow freely between them from any 
direction. The thermometer is placed with its bulb in the louvre 
box, and its readings taken through an opening g, in the draught 
tube, this opening being closed with a glass plate or provided 
with a door. In this screen the wind blows freely over the 
thermometer, and when the wind falls and the sun shines the 
current is kept up by means of the draught tube, so that under all 
conditions nearly the true temperature is indicated. This screen is 
the last devised, and seems to possess over the other two forms the 
advantages of simplicity, cheapness, and convenience. 
These screens are suitable only for recording maximum tempera- 
tures. For minimum temperatures the arrangement is similar to that 
shown in figs. 3 or 4, only inverted, the draught tube being placed 
underneath the thermometer. A convenient arrangement is to use 
an L-shaped minimum thermometer, and fix the stem into the top 
shade. To enable the index to be set this shade is hinged, so that 
it can be placed vertically to cause the index to drop. 
Some of these instruments have been tested and found to work 
satisfactorily, but complete tests continued for some time in all 
weathers will be made and are required before any practical decision 
as to their value can be given. 
Wet Bulb Observations. 
In meteorology, the next most important point after the tempera- 
ture, is the humidity of the air. Different methods are in use for 
determining this most important point, but the one most generally 
used is the indirect one of simultaneous readings of wet and dry bulb 
thermometers. These observations are generally made in some kind 
of screen, the wet and the dry bulbs being placed near each other. 
Little consideration is necessary to show us that less favourable con- 
ditions for obtaining the true condition of the air could scarcely be 
selected, because the amount to which the wet bulb is depressed 
depends, not only on the dryness of the air, but also on the rate at 
which the wind circulates through the screen and over the wet 
surface. The result of this is, that with the same degree of humidity 
