328 Mr Galbi-aith's Barometric Measurements of Heights. 



torn of the height to be measured, otherwise great errors may 

 be committed in the determination of the final results, if the 

 barometer be changeable. 



I cannot illustrate this better, than by the measurement of 

 Ben Nevis, communicated in my last paper. On the morning 

 of the day preceding the day of ascent, on the morning of the 

 day of ascent, and on the evening after descent, from a number 

 of observations in the month of August 1830 : — 



On the 28th, at 6" 37" a. m. B = 29in.S87, r = 5r.O, t = 51°.0 

 29 ... 6 A. M. B = 29 .889, T =50.4, <=50.9 

 29 ... 8 15 p. M. B = 30. 112, T = 53 .0, < = 53 .0 

 29 ... 1 15 p. M. 6 =25 .466, r' = 37.7, < = 3r .0 



Now, on comparing the observations made on the 28th, at 

 6" 37™ A. M., with those on the 29th at 6'^ O"" a. m., and 

 gh \^m p jyj ^ it y^rjii \^Q found that the rise of the barometer 

 was tolerably regular, and that a mean between tlie two last 

 made on the morning and evening of the same day in which the 

 observations were made on the top at nearly equal intervals of 

 time, cannot be far from the truth. But if I had taken either 

 of these observations made at Fort William Inn, it is evident 

 that their height, resulting from a comparison with that on the 

 top, must have been erroneous to a considerable amount, either 

 in defector excess, according as I had adopted 29-889, or 30.112 

 for the term of comparison. 



Let B = 29.889, r = 50'. 4, t = 50°.9 



I = 25.466, t' = 37 -7, i' = 3T .0, then the resulting height will 



be 4270 feet. 



Again, let B = 30.112, r = 52'. 7, t = 53°.0 

 J = 25.466, t' = 37 .7, <' = 37 .0 



The height will be ... . 4476 feet. 

 The difference ^f these results is . 206 feet. 



And therefore the half or error of each, is 103 feet. 



From this examination, it appears that the difference of these 

 determinations is no less than 206 feet, giving a mean error of 

 103 feet for each. 



In hke manner, the sympiesometer observations made simul- 

 taneously with the above, will, 



By the first, give 4220 feet. 



By the second set, . ^ 4488 



Difference, 268 



Half, or mean error, . . . . 134 

 Or onh' differing 31 feet from the other, by an excellent mountain barome- 

 ter of the best construction. 



