VOL. LXVII.] PHILOSOPHICAL TRANSACTIONS. '245 



its variation is only a line and a halt"; and at Quito a single line. Now let us 

 suppose, that an altitude had been measured with the barometer at the level of 

 the South Sea, where the descent of quicksilver at the upper station was exactly 

 an inch in the mean heat of the day, answering to 84°4- On the former sup- 

 position of the weight of quicksilver to air, the height would be 1 3 1 00 inches 

 or 1091.7 feet. 



Hence/ ^9-^^" '^^^ - l69 = 29.761 1 _ ^^o 6 feet 

 Hence | ^s.ygc, §4^ - 169 = 26.76 1 J " ^^^'^ '®"' 



the logarithmic result, which is defective 201.1, or nearly ^W% parts. Now 

 this equation being divided between 2.45 the mean expansion of air, we have 

 nearly 92° for the difference between 844., the temperature of the observation, 

 and the zero of the scale, which reduces it to — 7% of Fahrenheit. 



Having now mentioned all the barometrical observations that have come to 

 my knowledge, tending any way to throw light on this very intricate subject, it 

 remains to sum up, from the whole, the general principles on which I have pro- 

 ceeded in constructing the table of equation for the heat of the air. It will be 

 remembered, that I have more than once remarked, that in the British observa- 

 tions, when the temperature was 52°, the defect was ^ -^ °'^ ^ ° , the lowermost 

 barometer standing at or near the level of the sea; but in the observations on 

 Tinto, a considerable hill appertaining to the third class, whose base is elevated 

 700 feet above the level of the Clyde at Glasgow, when the temperature was 

 52°, I found the equation to be little more than -tH-ct- Again, these two facts 

 being compared with the aggregate result of Mr. De Luc's observations, where 

 the lowermost barometer stood about 1300 feet above the sea, the equation for 

 the same temperature seemed not to exceed ttI-o- Lastly, these circumstances 

 being confronted with the results of Mr. Bouguer's observations, where the 

 lowermost barometer stood from 60OO to 8000 feet above the sea, the mean 

 equation for 55° was only -f-^^, which gives ,l l„ for the heat of 52°. Now 

 these Peruvian observations, which I believe to be exceedingly good, from the 

 steadiness of the barometer in that part of the world, being substituted instead 

 of those not yet obtained in our own quarter of the globe, there seemed to be a 

 necessity for concluding, that the equation for middle latitudes, with any assigned 

 temperature above or below the zero of the scale, diminished as the height of 

 the place above the sea increased; which consequently implied, that the magni- 

 tude of the logarithmic terms increased faster than the dilatations of the air. 

 But when the comparison was carried yet further, and the observations in Peru 

 and at Spitzbergen were fairly brought into one view, there appeared to be suffi- 

 cient grounds for suspecting, if not absolutely for concluding, that there could 

 be no fixed zero for the scale depending on the temperature of the air; but that 

 it would change with the density of the atmosphere appertaining to the latitudes. 



