20 On the Height of Mountains, Headlands, <Sfc. 



the ultimate result. It is well established, that the height of an 

 homogeneous atmosphere whose density would be equal to that 

 of the air at the earth's surface, and weight the same as that of 

 the real atmosphere when compared with a column of mercury 

 or any other fluid of the same weight, the heights will be recip- 

 rocally as the specific gravities of the air and mercury or other 

 fluid. So that, if we take the specific gravity of the air at the 

 earth's surface at li when compared with distilled water at 1000, 

 and that of mercury at 14000, also the column of mercury in the 

 barometer =29£ inches, we have 

 li : 14000: :29£ ; 344166 inches =28680.5 feet=5.43 miles. 



But the specific gravity of fluids varies as their temperatures 

 vary. It has been found by various experiments, that when the 

 mercury in the barometer stands at 30 inches, and the thermome- 

 ter at 55°, the specific gravity of air, water and mercury are nearly 

 as 1}, 1000 and 13600. Hence, 



li : 13600: : 30 : 340000 inches = 28333 1 feet =5.366 miles, 

 the height of a homogeneous atmosphere. Again, taking the spe- 

 cific gravity of the air at the earth's surface at If, which some 

 affirm, and the barometer at 29£ inches, it will be 



If : 13600: :29£ : 328255 inches =27358 feet = 5. 18 14 miles. 

 Hence, generally, we may assume if the air was of the same 

 density at all altitudes as at the earth's surface, its height would 

 be between five and six miles. But it matters not what degree 

 of temperature we assume, for we can always accommodate the 

 result to any other temperature, as before observed, by augment- 

 ing or diminishing the result by the j^th part for every degree 

 above or below 31°. 



It may further be observed, that the common barometer is the 

 best and most to be depended on ; for many which are said to be 

 improved, have only the recommendation of deviating from it in 

 simplicity. It appears from accurate observations, that mercury 

 stands higher in tubes of a larger than in those of a narrower 

 bore ; and therefore, when observations are made with different 

 barometers, attention should be paid to the difference of their di- 

 ameters. In order to prevent the effects of the attraction of co- 

 hesion, the bore of the tube should not be less than one fourth 

 of an inch ; but one third of an inch would be better. 



We cannot conclude this problem better than by giving a re- 

 mark of Prof. Dalby's, whose practice in trigonometrical survey- 

 ing was extensive. 



