ALTITUDES OF STATIONS IN MAEOCCO. 361 



recommences. Even as regards stations near enough to be 

 within sight of each other, repeated observations, however care- 

 fully corrected, give sensibly different numerical results, and 

 when the stations are widely separated the discrepancies become 

 serious in amount. The best course for a traveller in a. moun- 

 tain country is to endeavour to ascertain as nearly as possible 

 the altitude of some fixed station by taking the mean of several 

 observations compared with his distant station, and then to 

 determine the altitude of the higher points reached near to such 

 fixed station by comparison with an assumed reading of the 

 barometer at the latter as derived from intercalation. 



The altitudes of the stations at Hasni and Iminteli, given 

 in the followiag table, derived from several comparisons with 

 the Mogador readings, are probably nearly correct. That of 

 Arround, as derived from comparison with Mogador at a time 

 when the oscillations of pressure were relatively great and 

 rapid, does not deserve much confidence ; and the mean of two 

 comparisons with Hasni has been preferred, the more readily 

 as this nearly agrees with the ■ result obtained from a boiling- 

 water observation. 



For the reduction of our observations the formula proposed 

 by Count St. Robert, and first published in the Philosophical 

 Magazine for 1864, has been preferred, and, for convenience, 

 the tables based on that formula, published by the same author in 

 the Memoii's of the Academy of Turin for 1867, have been used. 



It is true that in the construction of the latter tables a value 

 has been assumed for the constant expressing the rate of dimi- 

 nution of density in the atmosphere corresponding to uniform 

 increase of altitude that is not constantly correct ; but it would 

 appear that the error resulting from this is but trifling. In 

 regard to the greatest elevation attained by us in the Atlas, the 

 difference in the measurement obtained by using the tables from 

 that ascertained by accurate computation from the formula does 

 not exceed 5 metres. 



It may here be remarked that the altitudes inserted in some 

 letters from Sir J. Hooker to the late Sir Roderick Murchison, 

 which were published in the Proceedings of the Royal Geo- 

 graphical Society for 1871, and also most of those given by Mr. 

 Maw in a paper presented to the Geological Society in January, 

 1872, were roughly calculated at the time when the party were 



