24 THE VOYAGE OF H.M.S. CHALLENGER. 



Fort Rae, lat. 62° 39' N., long. 115° 44' W., illustrates this peculiarity of the diurnal 

 pressure (Plate I. fig. 15). 



Over the ocean in high latitudes the diurnal curves of pressure show only one 

 maximum and one minimum, but the times of their occurrence are directly opposite 

 to those over land. 



It is evident that in employing the data of Table IV. in " correcting " for daily 

 range, with the view of bringing the observations to the true daily mean pressure, 

 the greatest care is required in selecting stations whose means will give approximately 

 true " corrections." Indeed, as regards narrow steep valleys, any such attempted 

 reductions can at best only be regarded as useless. 



The daily oscillations of pressure at places given in Table IV. show the same feature 

 to be apparent even in comparatively shallow valleys bounded by distant rising grounds 

 with low surface gradients. This consideration must not be lost sight of in any effort 

 to trace the simple temperature effect on the daily barometric tides. In truth, the 

 observed temperatures made at the station can be used in such a discussion only when 

 the observations are made on the open sea, or on what is substantially an open plain at 

 some distance from the sea. On coasts, in comparatively narrow valleys, but in a less 

 degree on peaks, the problem becomes very complicated, and in attempting to solve 

 it the temperature of the region for some distance round the place of observation 

 must also be taken into account. 



Towards the end of Table IV. are given the diurnal ranges for Polar Stations, 

 including nearly the whole of the International Arctic and Antarctic Stations during 

 1882 and 1883. An examination of these is sufficient to show that several results must 

 be accepted with some reserve as a representation of the facts of the diurnal variation of 

 pressure in these higher latitudes. More might have been made of these observations if 

 they had been published as made, that is, if, instead of reducing to 32°, by the methods 

 in common use, the original readings of the barometer and of the attached thermometer 

 had been printed. Since the daily range in these regions is very small, probably not 

 exceeding 0-010 inch, and since in every case when the temperature shown by the 

 attached thermometer differs from that of the barometer taken as a whole, it follows 

 that for every degree of difference the reduced observations contain an error of about 

 0-003 inch. Indeed, the hourly pressures at several Arctic Stations, instead of showing 

 the horary changes of pressure, appear in some cases to indicate in an obscure way the 

 changes of temperature, artificial or otherwise, of the apartment where the barometer was 

 hung. In those cases where care has been taken to secure that the monthly means of 

 the attached thermometers, for the different hours of the day, represent the temperature 

 of the whole barometer to within a degree, the results show the extension of the oscilla- 

 tions into Arctic and Antarctic regions. They are probably dependent on the diurnal 

 changes in the temperature of the air itself, irrespective of those of the earth's surface, and 

 they may be, in some way, influenced by quasi-tidal movements from lower latitudes. 



