144 Of the State of Vapour 



mofphere being fomewhat lower, as Mr, Watt's experiments 

 prove. 



Mr. Watt difcovered that the latent heat of fteam dimi- 

 niflied in proportion as its fenfibleheat incre^fed, Phil. Tranf. 

 1784, p. 335. Now the fenfiblc heat of (ieam exceeds 180° 

 above the freezing point when the barometer flands above 

 thirty inches, and lefs than 180' w^hen the barometer fiands 

 lower than thirty inches, as Mr. De Luc lirft difcovered, and 

 may be feen in Sir George Schuckburgh's and Mr. De Luc's 

 tables, Phil. Tranf, 1779, p. 375. Vxom theie I have de- 

 duced the following table : 



The accuracy of this table even in the 

 lower part of the fcalc is fufficiently apparent 

 by the refult of the experiments of Sauiiure on 

 ebullilionon Mount Blanc ; for on that enor- 

 mous mountain, ihe barometer ftanding at i6 

 French inches or 17-05 Engliih, he found wa- 

 ter to boil at the heal of 68*993 'of Reaumur, 

 a degree which on Geneva thermometers is 

 equal to I'^^-^G^ Lngliih. — Hence we ice that 

 diltiilation may be more advantageoiifly ef- 

 fected on mountains than on plains, and at 

 low barometrical heights than at the creater, 

 yet within certain limits ; for, at heights that 

 fm-pafs 8 or JO tlioufand feet, the fuel, by 

 rcafon of the rarity of the air, is more flowly 

 confnmcd. Hence aifo, from the knowledge 

 of the degree of the heat of ebullition to two 

 or more decimal places, the (late of the baro- 

 meter above or below 213'" may be infeired to 

 one or more decimal places. The rcafon of this 

 rapid diminution of the heat of e'niliilion be- 

 low 25 inches is evidently the diminution of 

 refiftance, from the diminifhed weight of the atmofphere, 

 which then is very fenfible : but as the cold continually pro- 

 duced by evaporation is then alfo very confiderable, the time 

 nccefTary to procure ebullition is longer, as SaufTure remarked 

 on Mount Blanc. Vol. vii. in 8vo, § 201], p. 328. He 

 found the heat of ebullition barometer 16 to be 68*993 de- 

 grees, or in Enghfli meafures barometer ■I7"05. 185-5 ^^ 

 Fahr. (counting one of Picaumur at Geneva ~ 2225 of 

 Fahr.) 



Hence fince, according to Mr. Watt, the fenfible heats of 

 the vapours of boiling water at ditferent barometrical heights 

 are as the barometrical heights reciprocally, and the fpecific 



heats 



