244 PHILOSOPHICAL TRANSACTIONS. [aNNO 1777. 



equation tVut?' ^o ^e added to the logarithmic result, to obtain the true height 

 in Britain when the temperature is 42", there is an access of -rJ-^: and, instead 

 of the usual ratio of the weight of quicksilver to columns of air, of equal alti- 

 tude and temperature in Britain, namely about 11200, we have that of 

 10224 to 1. Thus air at Spitzbergen seems to be specifically heavier than that 

 affected with the same heat and pressure in the middle latitude: whence it follows 

 that, instead of 32° wliich is found to be the zero of the scale about the middle 

 of the temperate zone, we shall have -j*^ = \Q°.2 + 42° = 6l°.2 for the zero 

 at Spitzbergen, within 10° of the North Pole. By attending diligently to 

 what Mr. Bouguer has told us of the steadiness of the barometer throughout 

 the year in Peru; the uniformity of the mean temperature in every assigned 

 station; and his mode of computing, by means of the tables of common loga- 

 rithms, the altitudes of the Cordillero mountains above the valley that extends 

 itself between them ; it will be no difficult matter to discover, nearly at least, 

 what sort of equation became necessary; and what were the relative weights of 

 quicksilver and air of the mean temperature, not only in that high region of the 

 atmosphere, but also at the level of the sea. Thus, by inspecting the table of 

 computations, it will appear, that columns of air, whose bases were removed 6 

 or 8 thousand feet from the level of the sea, and whose heights equalled that 

 distance, when the temperature was 55° of Fahrenheit, as determined from the 

 mean between the coldest of the morning and hottest of the afternoon, the 

 mean logarithmic defect was only -fSiA- whereas, in measuring heights near the 

 level of the sea, in middle latitudes, the common equation for that temperature 

 is ~ll-^. The mean ratio of the weight of quicksilver to air, on these long 

 columns comprehended respectively between Carabouro and Quitou, and the 

 summus of Pichincha and Coraqon, is that of 16792 to J. On the altitude of 

 1534 feet, intercepted between Carabouro and Quito, which short section of 

 the column is about half-way between the level of the sea, and the summits of 

 the Cordilleros, the mean temperature being 66'^, the ratio is that of 1 5O89 to 

 1 : hence it seems probable, that quicksilver would have to the difterent sections 

 of the general column of air, comprehended between the top of Coraqon, 

 nearly the following ratios: 



Temp. 



At the level of the South Sea 8+J 13100 to 1 



Half-way from thence to Carabouro 75|. ... 14100 



A.t Carabouro 6'6| 15100 



Half-way from thence to Cora9on .- 55 .... 16100 



At the summit of Coraqon iSh .... 17 100 



The mean of which is 65 15100 



Mr. Bouguer tells us, that the barometer in the torrid zone varies not at the 

 sea shore above 2i or at most 3 lines throughowt the whole year. At Popayan, 



