liv INTRODUCTION TO THE MAKERSTOUN OBSERVATIONS, 1845 anv 1846. 
moveable front d of the wooden case, 4 feet above the soil; the bulbs project below 
the wooden slab ¢, and as holes are cut in the wooden case behind them, they are 
exposed to freely-circulating air. The wooden 
case, which has slightly-projecting top and sides 
at the front, and a double sloping back, revolves 
on a post f, and can be turned from within the 
Observatory by means of cords and pulleys g g. 
When an observation is made, the case is turned 
till the thermometers face the window h, being 
9 inches distant from it ; after reading, which is 
done through the glass (thus avoiding any error 
due to proximity of the observer, or the light at 
night), the case is again turned with the back 
towards the window, or towards the wind if it 
rain. It was found early in the summer of 1843, 
that in spite of the precaution of turning the back of the case towards the sun 
before 7" A.M. and after 5" p.M., if the sun shined brightly, the temperature indi- 
cated by the thermometer was visibly increased. [In all such cases, therefore, the 
moveable front d was lifted off the case and suspended in the shade, at an equal 
height from the soil, on the west or east wall of the Observatory, being kept apart 
from it by projecting knobs. Observations at different times shewed, that, all 
other things being equal, the temperature was the same in all the three positions, 
but when the sun shined on the case, it might be one or two degrees less to the east 
or west than to the north. The observations made to the east or west are indicated 
in the column of differences by a cross, thus }, for the first observation after re- 
moval from the case, and by a cross, thus |, for the last observation before replacing 
the thermometers on the case. 
90. It sometimes happens, when the air is very humid, during frost, and on 
clear nights, especially when the temperature is falling, that the dry bulb thermo- 
meter reads less than the wet bulb ;* when such is the case, the diference of the 
readings of the two thermometers has not been given, and in the summations for 
* This apparent anomaly in frosty nights, it is conceived, is due to the deposition of moisture 
on the silk coyer of the wet bulb, which is frozen as it is deposited, till it becomes a thickish coat 
of silk and ice; the dry bulb receives, at the same time, a thin coat of moisture, and becomes a 
more facile wet bulb, In clear, humid nights, without frost, nearly the same explanation will apply ; 
the dry bulb will radiate its heat into space with more facility than the wet bulb. It might be 
preferable, therefore, on these occasions, to make use of the readings of the wet bulb for the tem- 
perature of the air, and of the readings of the dry bulb for the temperature of evaporation during 
frosty nights; where, however, the differences of the readings may be considered due chiefly to the 
different radiating powers of the two bulbs, the readings, perhaps, should be considered the same ; 
this has been done in all cases in the present volume. 
