VOL. LXVII.] PHILOSOPHICAL TRANSACTIONS, '241 



with the manometrical expansion. In this case, the ratio of the weight ot 

 quicksilver to air is as 11377 to 1; greater very considerably than 11232 to 1, 

 assigned to them by Mr. De Luc, when the temperature is 6g°.32, answering 

 to the zero of his scale, without any allowance for the diminution of pressure 

 on his columns, whirh should have rendered air still comparatively lighter. 

 From the British observations, made on the most considerable heights, it 

 appears, that when the temperature of the air is 28°.2, the ratio of its 

 weight, with respect to that of quicksilver, is as 1 to 10552: hence the increase 

 of the weight of air, on every degree of difference of temperature between 

 28°.2 and 52°.5, amounts to 34.4; and hence we have 52''.5 ~ 4°.2 = 48°.3 for 

 the temperature of the air in Britain, when its weight would be -t-t^tt of that of 

 quicksilver; and consequently agree with Mr. De Luc's, though the heat would 

 differ from his 21°. It will no doubt be remarked, that the equation for the air, 

 resulting from the operations of the barometer, falls short of that given by the 

 manometer. Part of the difference may arise from the small number of barometrical 

 observations obtained in extreme temperatures. I shall however adduce reasons 

 hereafter for supposing that it really should diminish, because of the drier and 

 less elastic state of the superior air, compared with that taken into the manometer 

 at the earth's surface. In the mean time, since both instruments agree in the 

 equation for 52° which is a heat that the barometer will very frequently be used 

 in, it seems best to adhere to the mean manometrical result 2.45, in fixing the 

 zero of the scale, which is obtained in the following manner. 



Divide the excess or defect, expressed in 1000th parts of the logarithmic 

 result, by 2.45, the mean expansion of air for each degree of the thermometer; 

 the quotient will give the number of degrees, in the first case to be added to, 

 and in the last subtracted from, the temperature of the air in the observation ; 

 the sum or difference answers to the zero of the scale, or that temperature when 

 the logarithmic result gives the real height in English fathoms and 1000th parts. 



According to this mode of computation, we have, from the aggregate of the 

 several classes of British observations, the place of the zero as follows: 



By tlie first class of observations in and near London 1 „ . „ , . 



between the temperatures of / 2^ .5and^ l°.2at32°.2 



2d, near Taybridge 46. 1 — 62.9 - 3 1 . 1 



3d, near Lanark 4.4. g^. - 32.8 



4th, near Edinburgh 17. 70.7 _ 3] 3 



5tJi, near Linhouse 26.1 46.5 - '20,Q 



6th, near Carnarvon 49. 1 — 62.3 - 32.Q 



Mean place of the zero at 3 j 7 



The number 31°.7 difl^ering so very little from 32°, we may hereafter consider 

 that remarkable point of Fahrenheit's thermometer, as the zero of the scale 

 depending on the temperature of the air; and hence is deduced the 2d part of 



VOL. XIV. I I 



