Defcription of a new Hygromiter and Pholomekr. 463 



complete, and for the fpace of two jrears I frequently employed it in meteorological obfer- 

 vations and other refearches. I was ftill diflatisfied, however, with its complex nature, 

 and the attention required in ufing it. Another refource prefented itfelf in the augmented 

 clafticity which air acquires in diflblving moifture. To meafure that efte£t, I contrived a 

 fimple machine which yet anfwered perfedly the end propofed. And though, in its origi- 

 nal objeft, it has been fince fupcrfeded, I found it of the higheft utility in detedling thofe 

 minute alterations of volume which take place in the union of various bodies, thus extend- 

 ing the bounds of corpufcular dynamics, and marking the alliance with philofophical che- 

 miftry. 



In the fevere winter of 1795, 1 was naturally tempted to make experiments on the eva- 

 poration of ice, and the cold thereby produced. Inftead of fixing the thermometer in a 

 block of ice, I had the bulb covered with a congealed cruft, by repeatedly fprinkling it with 

 water, and fuffering this to freeze. Placing in the fame fituation another correfponding 

 thermometer with a naked bulb, I was furprifed to remark how quickly and fteadily their 

 interval reached its maximum, extent, the evaporation of a minute film of.ice, proving fuffi- 

 cient to cool down to its proper ftandard the whole mafs of included quickfilver. Of the 

 legitimacy of this inference I was convinced, when I refle£led on the vaft coiifumption of 

 heat which mud take place during the tranfition into the gafeous form, of a comparatively 

 fmall portion of evaporable matter *. Two thermometers, therefore, filled with any ex- 

 panfible fluid, with quickfilver, alcohol, or air, the bulb of the one being wetted and the 

 other dry, will by their difference denote the flate of the air in refpe£l; to humidity. No- 

 thing was wanted but to combine thofe inftruments in fuch manner that they fliould indi- 

 cate merely their difference of temperature. To accomplifh this objedV, it fortunately oc- 

 curred to me to employ two hollow glafs balls, communicating with each other by a narrow 

 tube, which contained fome coloured liquor. In ordinary cafes, the intermediate liquor 

 would continue ftationary; for the air in both balls having the fame temperature, and con- 

 fequently the fame elafticity, the oppofite preffures would precifely countera£l each other. 

 But if, from the a£lion of the external air on the moiftened furface, the one ball became 

 colder, it is manifeft the liquor would be puflied towards it by the fuperior elafi;icity of the 

 air included in the other, fo as to mark by the fpace of its approach the depreffion of temr 

 perature induced by evaporation f. This contrivance fucceeded admirably, and after re- 

 peated trials I fixed on the moft fuitable form and dimenfions. In the courfe of thofe 

 trials, I was led to the difcovery of the photometer, which (hall be hereafter defcribed. A 

 farther alteration was then made in the conftruftion of the inftrument, and I was anxious 



• A cold of one degree centigrade is produced in a mafs of water by the evaporation of lefs than the 

 500th part of its weight. 



f This fuppofes that air expands uniformly with equal additions of iieat, a pofition not indeed ftriftly 

 accurate, but which differs much lefs fropa the reality than is fometimes alledged. Experimenters are not 

 even agreed whether thofe expanfions form a rifing or a defcending feries. At any rate, in the narrow 

 range of the inftrument, that deviation from regularity becomes altogether infenfible. 



3 O 2 to 



