465 



remaining parts being not then required. The apparatus is furnished 

 with two or three yards of vulcanized India rubber tube of the smallest 

 bore, one end of which perforates a cork nicely fitted to the slightly 

 tapering silver funnel through which ether is supplied to the silver 

 cylinder ; to the other end is attached a small stopcock, connected with 

 a large flaccid bladder, or air bag, such as is commonly used for gases. 

 The Hygrometer, thus fitted up, now constitutes an independent instru- 

 ment, which may be used as follows : — Some ether (lightest) being 

 poured in through the funnel, the cork holding the tube is to be inserted, 

 and the instrument fixed in any convenient situation ; the observer, 

 stationed at a distance of several feet, places the inflated bladder under 

 his arm or foot, and, moderately pressing it, regulates the passage of 

 the air by the stopcock ; and through a small telecope watches the effect 

 until the first cloud appear on the silver. The telescope renders the 

 degrees of the Thermometer, and the height of the mercurial column, 

 distinct to even the eighth* of a degree ; but the distance of the observer 

 must be limited to that at which the first invasion of the tarnish on the 

 silver can be perceived : for my own part, with a telescope, I could 

 barely perceive the incipient cloud at the distance of eleven feet, in a 

 most favourable light. 



The action of the bladder is precisely the same as that of the 

 aspirator or syringe ; it is easily inflated, when the stopcock is removed, 

 by a few expirations of breath. 



Regnault was able to make several determinations of dew-point 

 with his Hygrometer, in which ' ' he obtained results perfectly identical 

 in succesive determinations. The experiments were made in a large 

 ampitheatre, of which the temperature andhygrometric condition changed 

 very slowly." In my trials I could obtain but few identical results 

 within an hour ; many of them were made in the months of February 

 and March, 1869, memorable for rain and storms; in the open air, 

 during a calm, the indications scarcely varied half a degree in successive 

 experiments during an hour. Indeed, under equal circumstances, it is 

 difficult to see why there should be any variation ; at the proper tem- 

 perature, moisture will precipitate from the air, and bright silver will 

 become dull without fail. The only source of uncertainty is the visual 

 capability of the observer at the proper distance. 



We come now to consider the use and application of the Psychro- 

 meter, or wet bulb Thermometer. The chief point to be attended to, 

 in order to determine the dew-point and fraction of saturation, is the 

 true depression of the mercury effected by the evaporation of water 

 from what may be called the bulb. If the instrument be used by 

 merely wetting the cambric envelope, and leaving it to spontaneous 



* Regnault reads off hi3 Thermometer to one-tenth of a degree ; but that is centi- 

 grade. The centigrade is to Fahrenheit as 1 8, 8 is to 1°. 



