VOL. LXVII.] PHILOSOPHICAL TRANSACTIONS. 239 



line of collimation of the instrument, which was found to alter in carrying, 

 might occasion no error, one or more of the angles of elevation, at each station, 

 were taken on the arc of excess, as well as on the quadrantal arc. In all cases, 

 the usual* allowances were made for curvature and refraction: and for the cor- 

 rection of the last, sometimes the angles of depression as well as of elevation 

 were taken. When time would permit, the geometrical operations were repeated 

 at the first stations; or the angles of elevation were observed from some new- 

 point connected with the first, and whose relative height, with respect to the 

 others, was known. Small altitudes were occasionally determined by levelling 

 from one station to the other. 



I now proceed to give some account of the barometrical observations. The 

 heights in and near London being so very inconsiderable, it was easily foreseen, 

 that nothing conclusive could be drawn from observations made on them alone. 

 It was however natural enough to try, even on these, whether the rule we had 

 been furnished with would answer.'' A small height of 41 feet 4 inches, which, 

 without inconveniency, could be recurred to at all times of the day, and all sea- 

 sons of the year, was the first that was made use ot". St. Paul's, Hampstead, 

 Kew pagoda, and Shooter's-hill, were the next. The mean results of many ob- 

 servations on the first 3, and of several on Shooter's-hill, were found to be defec- 

 tive. In general the coldest observations, made in the morning and evening, 

 when the temperatures at the 2 stations differed least from each other, answered 

 best. In the hottest part of the day, when that difference was the greatest, the 

 results were most defective. Some months spent in Scotland in the summer of 

 1774, afforded opportunities of making barometrical observations on hills of va- 

 rious heights, from 3 or 400 to upwards of 3000 feet, as exhibited in the pre- 

 ceding list. That season was remarkably cold and wet; therefore, in these ob- 

 servations, the mean temperature of the air in the shade was commonly about 

 53°. The hottest never exceeded 63° in the plain ; and the coldest, namely 

 those on the highest mountains, were generally from 43° to 48°. 



From the defect found in the results of these observations, which, with res- 

 pect to temperature, correspond to the mean and hottest of those made at sun- 

 rising on Saleve, and without any exception whatever, I could easily discover, 

 either that a much greater equation than what the rule directed, must be applied 

 for each degree of heat above the zero of the scale; or, that the zero itself 

 would fall considerably lower than 39°74, where Mr. de Luc's formula, adapted 



• If the square of die distance be divided by the diameter of the earth, tlie quotient will give tLe 

 curvature of the globe on that distance, or die excess of the apparent above the true level : and, by 

 Mr. Maskelyne's mle, the square of the distance being divided by the diameter of the earth, aug- 

 mented by ^ part, we have die allowance for curvature and refraction ; which last is supposed to raise 

 (he object, by an angle equal to that of a great circle subtended by ■— part of the distance. — Orig. 



