226 Canadian Record of Science. 



correct of the many widely-divergent numbers given in 

 this connection. 



With respect to the determination of altitudes by means 

 of the barometer, a few words may not be out of place, 

 and the following, from Johnson's Theory and Practice of 

 Surveying (Ed. 1887, p. 128), may be quoted : 



" It (the aneroid barometer) has a vernier attachment, 

 and is read with a magnifying-glass to single feet of eleva- 

 tion. It must not be supposed, however, that elevations 

 can be determined with anything like this degree of accu- 

 racy by any kind of barometer. The barometer simply 

 indicates the pressure at the given time and place, but for 

 the same place the pressure varies greatly from various 

 causes. All barometric changes, therefore, cannot be 

 attributed to a change in elevation, when the barometer is 

 carried about from place to place. 



" If two barometers are used simultaneously, which have 

 been duly compared with each other, one at a fixed point 

 of known elevation and the other carried about from point 

 to point in the same locality, as on a reconnoissance, then 

 the two sets of readings will give very close approxima- 

 tions to the differences in elevation. If the difference of 

 elevation between distant points is desired, then long 

 series of readings should be taken to eliminate local 

 changes of pressure. The aneroid barometer is better 

 adapted to surveys than the mercurial, since it may be 

 transported and handled with greater ease and less danger. 

 It is not so absolute a test of pressure, however, and is 

 only used by exploring and reconnoissance parties. For 

 fixed stations, the mercurial barometer is to be preferred." 



The observations on the trip were made with two ane- 

 roid barometers, one manufactured by Cary, of London, 

 and the other by Usteri-Eeinacher, of Zurich. The 

 instruments were carefully compared with the standard 

 mercurial barometer in the Observatory at McGill College, 

 where readings were made throughout the day, and these 



