No. 21. VERIFICATION OF REGNAULT'S FORMULA FOR THE WET AND DRY BULB THERMOMETERS.

THE apparatus arranged for this purpose is shewn in the figure (fig. 9). A, B, and C are three glass globes each pro

To Aspirator

B

W

Fig. 9.

vided with three necks; the globes are mounted on stands so that the middle neck of each opens vertically. In the first is placed a Regnault hygrometer R, with its thermometer T. The current of air for cooling the ether is provided by a wateraspirator of the Magnus pattern, which has proved itself to be very convenient for the purpose. The silver thimble of the hygrometer is near the centre of the globe, and the observer can view it as closely as he pleases without danger of interfering with the deposit. It is generally found that the deposit is most easily visible if the direction of the light which illuminates the thimble be inclined to the line between the thimble and the observer at an angle rather less than a right angle.

Between A and B is a coil of copper-tubing K. This is intended to bring the air which has been cooled by passing over the cold surface of the thimble back to the temperature of the air.

In B is a dry bulb thermometer T,, intended to determine the temperature of the air. In C is a wet bulb thermometer T.

The bulb is covered with muslin and kept moist by means of a wick which passes through glass tubes to the water bottle W.

Air is made to pass through the whole set of bulbs by the use of foot bellows; it may be sent through in its natural state, or by interposing a calcium-chloride bottle or a bottle of pumice moistened with water it may be delivered either dried or moist.

The formula for the reduction of the wet and dry bulb observations is taken from Jelinek's Anleitung zur Anstellung Meteorologischer Beobachtungen, and is Regnault's formula altered in shape. It is the basis of Jelinek's Psychrometer tables for temperatures of the wet bulb above the freezing point. The formula is

e" e'0009739t' (t-t')-5941 (t-t')

=

It turned out as was to be expected, that the temperature of the wet bulb depends upon the rate at which the air is driven through the apparatus, the variation being something like a degree for ordinary atmospheric states.

The results obtained give the observations of the lowest temperature of the wet bulb.

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