18 Mr William Galbraith on the Tides and Dew- Point. 
moderate means. It is clear, from a comparison of the dew-point 
temperature with that of the atmosphere, as they approached to 
equality, rain certainly followed. 
Note 1.—It has often struck me how much useful matter might 
be obtained by gentlemen possessing pleasure yachts, provided they 
had a little taste for, and knowledge of branches of useful science, 
instead of wasting their time in indolence, or at most, pleasant 
amusement. 
Note 2.—On the Height of Mont-Blanc.—In the preceding paper 
I have shewn the method I pursued in determining the height of a 
given point above the mean level of the sea, whence my heights in 
Arran were derived trigonometrically, and a like plan must be fol- 
lowed in all similar cases claiming the requisite accuracy. In this 
way, too, a series of levels was carried from the Atlantic Ocean, 
through France towards Switzerland, from which the height of Mont 
Blanc was determined, as shewn in the Description Geometrique 
de la France, by the late M. Puissant. 
From data contained in these works, I gave a computation in the 
new edition of Ainslie’s Surveying, in which I regret there are one 
or two typographical errors, but not in the final results, which may 
be considered correct. Since then I have revised the whole, and 
find the mean from Plana’s observations to be 15817:86* feet above 
the mean level of the sea; Colonel Corabceuf’s give 15788°05 feet, 
forming a mean of 15802°96 feet, nearly what I formerly gave, with 
a difference of 29°81 feet, and the half of which, about 15 feet, may 
be reckoned the probable error in the final result, as in Johnston’s 
Physical Atlas. 
I may remark, however, that Mrs Somerville, in her Physical 
Geography, vol. i., p. 67, edition of 1849, gives 15759°8 feet. 
Again, in her Appendix, vol. ii., p. 418, she gives 15739 feet, on 
the authority of the Piedmontese Surveys, published in 1645, and 
Eichman’s Swiss Surveys in 1846. 
This shews the difficulty of determining accurately the heights of 
mountains of great altitude. 
Professor Forbes gives, in his book of Travels through the Alps, 
15744 from the French engineers. 
The mean of all these will give about 15760 feet for the culmi- 
nating point of central Europe, though I prefer 158038, the mean of 
Plana and Corabeouf’s results. 
* This differs from Plana’s own results, because the heights of Mont Colom- 
bier and Mont Granier had not been then definitely fixed, and were 8-81 metres, 
or 29 feet, too small, as has been latterly found by the French engineers. 
