STATE OF VAPOUR SUBSISTING IN THE ATMOSPHERE, 



Heat o f 

 boiling 

 water. 



Bar. 



30 

 29 



28 



27 



26 



2.5 

 94 

 25 



22 

 21 

 20 

 19 

 18 

 17 

 16 

 15 

 11 



Heat. 



212° 

 210,28 



208,52 

 206,73 

 204,91 

 203,06 

 201,18 

 199,27 

 197,33 

 ]95,36 

 193,36 

 191,06 

 188,46 

 1 85,56 

 184,36 

 180,86 

 176,70 



209 



The accuracy of this table even in the lower Table of the 

 part of the fcale is fufficiently apparent by the re- h^Vingvvltsr°at 

 fult of the experiments of Sauffure on ebullition on different irations 



,,_,,„, . of the barometer, 



Mount Blanc ; for on that enormous mountain, the f rom , i ncft e» 

 barometer (landing at 16 French inches or 17,05 t0 i4 inche8 « 

 Englifh, he found water to boil at the heat of 

 68°, 993 of Reaumur, a degree which on Geneva 

 thermometers is equal to 1 85°,56 Englifh. Hence 

 we fee that diftillation maybe more ad vantageouf- 

 Iy efFecled on mountains than on plains, and at low 

 barometrical heights than at the greater, yet w'th-i 

 in certain limits ; for at heights that furpafs eight 

 or ten thoufand feet, the fuel, by reafon of the ra- 

 rity of the air, is more (lowly confumed. Hence 

 alfofrom the knowledge of the degree of the heat 

 of ebullition to two or more decimal places, the 

 (late of the barometer above or below 212° may ' 

 infer to one or more decimal places.— The rea- 

 fon of this rapid diminution of the heat of ebulli- 

 tion below 25 inches is evidently the diminution of refiftance, 

 from the diminiflied weight of the atmofphere, which then is 

 very fenfible; but as the cold continually produced by evapo- 

 ration is then alfo very confiderable, the time neceffary to pro- 

 cure ebullition is longer as SaufiTure remarked on Mount Blanc* 

 vol. vii. in 8vo. § 2011, p. 328. He found the heat of ebul- 

 lition barometer 16 to be 68°,993 degrees, or in Englifh mea- 

 sures barometer 17,05. 185°, 5 of Fahr. (counting one of 

 Reaumur at Geneva = 2,225 of Fahr.) 



Hence (ince, according to Mr. Watt, the fenfible heats of Hence the fpe- 



the vapours of boiling water at different barometrical heights £ fic hea k c ,°£- 



* & ' . fteam at differ- 



are as the barometrical heights reciprocally, and the fpecific ent tempera- 

 heats of the vapours of water boiling are as the fenfible Heats tures# 

 reciprocally, it being known, that the fpecific heat of the va- 

 pour of water heated to 180 degrees above the freezing point 

 is 940. The fpecific or latent heat of the vapour of boiling 

 water, whofe fenfible heat is known, (and it may be known by 

 the barometrical height as fhewn in the above table and the 

 notes) may alfo be difcovered. 



Thus the fenfible heat of the vapour of boiling water ba- Inftance. 

 rometer 30 being 180° above the freezing point (212° — 32° 

 = 180°) and the fpecific or latent heat of vapour, whofe fen- 



Vol. V.— July. P iible 



