182 



Mr. W. Whitehouse on a New Hygrometer. [Feb. 22, 



For a bulb having one square inch of surface one drop per minute is 

 sufficient, though the time may range from 40 to 100 seconds without in- 

 convenience, the time being noted as the hydrated acid, after having 

 fulfilled its office, falls drop by drop from the bulb. 



The quantity of acid required at this rate is about 3 fluid ozs. per diem, 

 or one imperial pint per week, which is procurable of uniform density, 

 sufficiently pure and free from lead, at a cost of about 2\d. 



The temperature of the acid in the reservoir is of course that of the 

 surrounding air ; the elevation of temperature shown by the acid-bulb 

 thermometer is due to, and seems to be strictly a measure of, the amount 

 of moisture absorbed by the film of acid spread on the surface of the bulb, 

 say one square inch, continuously supplied in its concentrated state, and as 

 constantly passing off hydrated. 



While, therefore, this instrument is, like Mason's, intended to measure 

 the amount of hygrometric moisture in the air, and to do so thermome- 

 trically, it yet is, in its principle and in its operation, essentially of an 

 opposite character. 



The ordinary wet-bulb thermometer is at the zero of its scale in an 

 atmosphere of perfect saturation, and its action depends upon the amount 

 of sensible heat absorbed and rendered latent by evaporation of the water 

 from its surface. 



The acid-bulb thermometer is at its zero in a perfectly dry atmosphere, 

 and its action depends upon the amount of latent heat rendered sensible 

 by the condensation of vapour into water on the surface of the bulb, and 

 by the combination of this water with the concentrated acid. 



It would appear that an hygrometer on this principle is entirely free 

 from the action of frost ; while its sensibility is so great as to be at first 

 almost embarrassing. 



This may, however, be easily regulated and toned down, if necessary, to 

 any required range by the dilution of the acid with glycerine, a fluid which 

 is also of itself hygrometric, though its thermal effects are far less marked 

 than those of sulphuric acid. 



The following series of observations, made hourly and otherwise, at in- 

 tervals during the past few weeks, at the Meteorological Office, by the 

 kindness of the Director, will suffice to show approximately the relations 

 of the "acid" and the " wet bulb" respectively. 



They have been chiefly actual out-door observations, and have extended 

 over a considerable range of temperature and atmospheric variations. 



It will require a most careful series of observations to elicit all the points 

 noteworthy in the new instrument, and to determine the relative values of 

 the wet and acid-bulb readings, noting the behaviour of each at every part 

 of the scale, from absolute dryness to saturation, and at temperatures 

 ranging from 75° or 80° down to 0°. 



This will be necessary before the instrument can aspire to take its place 

 among the recognized standards of meteorological science ; but in the 



