ALTITUDES OF STATIONS IN MAROCCO. 359 



aneroids are exposed, a casual blow, received about the time of 

 landing at Mogador, caused the fall of 7 or 8 millimetres which 

 was then observed. It is quite possible that more complete 

 accuracy would have been attained by applying a correction of 

 — 1 mm. to M. Beaumier's observations ; but it was thought 

 more convenient to treat the discrepancy between the instru- 

 ments as altogether due to error in Seoretan's instrument, and 

 to apply to its readings in South Marocco the correction 

 + 7 '3 mm. So far as regards the altitudes determined by 

 comparison with the Mogador observations, the difference 

 between the method adopted and that above suggested is quite 

 insensible ; but with respect to the altitudes given in the fol- 

 lowing table as determined between April 29 and May 10, 

 wherein the barometric pressure at Mogador is assumed at 

 760 mm., it is clear that, if the error of Secr^tan's instrument 

 has been overcorrected to the extent of 1 millimetre, the alti- 

 tudes given in the table shovdd be increased by some 12 or 

 13 metres. 



The next corrections requiring consideration are those 

 arising from the temperature of the instrument at the time of 

 observation, and in reference to this point the best makers of 

 aneroid barometers are much open to criticism. They assert, 

 and- with approximate accuracy, that in the best instruments 

 compensation for the effect of temperature on the instrument is 

 provided ; but they forget that in order to compare the indications 

 of the aneroid with those of the mercurial barometer, or to apply 

 to them any of the formulte used for calculating altitudes, it is 

 necessary to know at what temperature the column of mercury 

 stands, the length of which is assumed to be shown by the scale 

 of the aneroid. In point of fact, the scale of the latter instrument, 

 when carefully laid down, is determined by direct comparison 

 with the mercurial barometer under varying pressures, and the 

 proper course would be to inscribe on the case of the aneroid a 

 record of the temperature at which that comparison was made. 

 From inquiries made of some of the best makers it seems pro- 

 bable that the best approximate correction is obtained by 

 assuming the reading of the aneroid to correspond with that of 

 the mercurial barometer at the temperature of 15° C, and this 

 has been applied in the annexed table. 



The height of M. Beaumier's instrument above the sea level 



