lvi 



Introduction to the Makerstoun Orservations, 1844. 



moveable front d of the wooden case, 4 feet above the soil ; the bulbs project below 

 the wooden slab c, 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 /, 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 A, 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 h A.M. and after 5 h 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 after July 9, 

 1844 are indicated in the column of differences by a cross, thus f, for the first ob- 

 servation after removal 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 difference of the 

 readings of the two thermometers has not been given, and in the summations for 



* The cause of tins apparent anomaly in frosty nights, it is conceived, is due to the deposition 

 of moisture on the silk cover of the wet hulb, 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 temperature 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. 



