108 Mr H. Meikle onjinding the Dew-pointy ^c. 



the gentleman remarks, p. 286, " that t^ f and B being given, 

 the actual weight of moisture contained in a given volume of 

 air, is nearly the same as is sufficient to saturate the same vo- 

 lume at the temperature f — (.062 {t — f) + 1.12 (30 — B)), 

 and under a pressure of 30 inches.'' 



This will be found a very convenient and pretty close approxi- 

 mation, when the actual pressure is 30 inches ; but, in other 

 cases, it may deviate widely from the truth, and yet nothing is 

 more easy than to render it alike correct for every pressure. To 

 accomplish this, we have only to cancel the superfluous term 

 1.12 (30 — B), which is at best of no use : for, when the actual 

 temperature ^, and dew-point f are given, the weight of mois- 

 ture in a cubic foot is in effect given, because it depends on no 

 other condition, being precisely the same whatever be the at- 

 mospheric pressure B. The experiments of Deluc, Dalton, 

 Gay-Lussac, Daniell, and many others, have completely settled 

 this point, and placed it for ever beyond any doubt. In this 

 our author seems to have followed Dr Anderson, in his habit of 

 making the barometric pressure an ingredient in almost every 

 formula and computation connected with the subject, and with- 

 out appearing to be aware that such procedure was quite incom- 

 patible with the theory of Dalton, to which he professed to ad- 

 here. Thus in the Edinburgh Encyclopedia^ Art. Hygrome- 

 TRY, p. 578, Dr Anderson, in giving a table of the force and 

 density of steam, says it is adapted to a pressure of 30 inches, 

 and that when the pressure is different, the numbers in the ta- 

 ble (none of which denote atmospheres but grains and inches), 

 must be altered in the same ratio as the pressure. Now, nothing 

 can be more obvious than that steam in vacuo has no concern 

 with the external pressure. But it is as well known that the 

 maximum force or density of steam in air has nothing to do with 

 the density of that air, being the same as in vacuo^ and yet Dr 

 Anderson almost every where assumes the force or density of 

 steam in air to be proportional to the barometric pressure. I 

 readily grant that whilst air undergoes a change of pressure, the 

 force and density of the vapour in it are altered in that ratio, 

 provided the temperature is not altered, and none of the vapour 

 has been liquefied ; but this is obviously a case essentially diffe- 

 rent from those to which I refer, where the pressure, of what- 



