and Levelling, <§r. 335 



Jardine, civil-engineer, by direct levelling, find his results, I 

 believe, differed little from the above, to which, as very near 

 the truth, I shall still adhere. 



As this height may therefore be considered well determined, 

 I was desirous of performing the measurement of it trigono- 

 metrically with great care by means of the best instruments 

 I possessed. That for angular measures was my altitude and 

 azimuth astronomical circle, and for meteorological observa- 

 tions Mr Adie's portable sympiesometer. As the thermome- 

 ter is a good one, the temperature must have been accurately 

 determined, and though its indications of pressure are not 

 cmite so accurate as those of a good mercurial barometer, yet 

 the differences will seldom exceed a few hundredths of an 

 inch when in a fair state of adjustment, which ought to be 

 repeated every two or three years. 



I adopt this instrument for all such operations, geodetical 

 and asti'onomical, in my travelling excursions, on account of 

 its being less liable to accidents than the barometer, and its 

 greater portability, while its indications are fully sufficient for 

 all ordinary purposes. 



This instance, then, affords me a fair criterion to judge 

 of the relative accuracy of direct levelling, of barometric 

 measurement, and of trigonometrical operations. When I 

 find, as will immediately appear, that I cannot avoid consi- 

 siderable discrepancies in my own residts, even by taking all 

 possible care, while, at the same time, I employ the best data 

 I can procure, combined with the co-efficient of terrestrial 

 refraction "by the formula just investigated, I shall then be 

 compelled to form a more candid and charitable opinion upon 

 the analogous operations of others. 



I. At my station on Inchcolm on the 15th of August 1810, 

 the observed zenith distance of the horizon of the sea down 

 the Firth of Forth to the eastward was 90 J 8' 21".2, while the 

 pressure of the atmosphere was 29.75 inches, and the tempe- 

 rature 64° Fahrenheit, the height of the station above the sea 

 will be determined by the usual formula 



rfA=i e (l +nf tan»D (A) 



in which g is the radius of curvature in the given direction, n 



