L. CASELLA S CATALOGUE 



by means of a camel-hair brush, with water as near 32 as possible. In such cases it is not 

 necessary to remove the ice from the bulb, but merely remove the drop which first forms from 

 the water, the temperature will then speedily settle so as to indicate the point of evaporation. 

 A piece of cotton- wick, well washed in clear soft water, is usually supplied with the instrument, 

 and used to cover the 'bulb instead of the muslin. In placing fresh covering on the bulb, it 

 should be loosened as is shown in the sketch above, and care taken not to restrict capillary action 

 by tying it beneath the bulb. 



The porcelain employed in all these instruments is of an improved kind, especially made for 

 L. CASELLA, and warranted impervious to all changes of the weather. 



TABLE or FACTORS by which the difference of readings of the dry bulb and wet bulb 

 thermometers is to be multiplied in order to produce the difference between the readings of the 

 dry bulb and dew-point thermometers. 



In order to obtain all the data deducible from the wet and dry bulb thermometers, Glaisher's 

 Hygrometrical Tables should be used. 



58. Daniel's Hygrometer 5 the thermometers divided on the stems, with ether test, 



complete in mahogany case (fig. 58) p. 21 . . 3 10 



This elegant instrument consists chiefly of a bent glass tube, with two balls a black one, 

 about one-fourth filled with the highest rectified ether. The stem incloses a sensitive thermo- 

 meter with the bulb placed rather below the centre of the ball, and a white one covered with 

 thin muslin, the interior of the tube being thoroughly deprived of air and the greatest care 

 observed not to over-boil or impoverish the ether. 



Directions for Using Daniel's Hygrometer. Turn the instrument up so that by applying 

 the warm hand to the covered bulb all the ether goes into the black bulb, then place it 

 upright as shown in the sketch, and pour ether upon the muslin enveloping the white ball, 

 and when sufficient cold is produced by evaporation of the ether from the black ball to 

 condense the moisture of the atmosphere upon its surface, in the form of a ring just below 

 the centre, the internal thermometer will show the exact temperature at which the deposition 

 of dew takes place, which is called the dew-point. 



59. Regnault's Condensing Dew-Point Hygrometer (Casella's Improved,) with ether 



bottle, etc., complete in mahogany case (fig. 59) p. 21 . 440 



*#* Agreeable to the suggestions of Colonel Sykes, F.E.S., and Dr. Miller, F.E.S., L. 

 CASELLA has adapted to this instrument a black glass bottle, with silver neck and tube, 

 which may be had instead of the silver bottle, or extra, at an additional charge of 20s. 



