44 .4. ]V. Greelij — Geograjjhy of the Air. 



latter is simply the measure of a resistance. The objection to 

 this form of determination is this, that it is a measure of mass 

 only. The density of two masses of air is determined to be the 

 same ; but as the density may result from two entirely different 

 causes, their physical relations cannot be fully expressed in units 

 of gravity. The methods of Professor Abbe and of M. Nils 

 Ekholm undoubtedly give good results, partly from the coinci- 

 dence that humidity usually varies directly as the temperature. 



The method proposed by Captain James Allen in 1888, which 

 is briefly described in apj)endix 24 to the annual report of the 

 Chief Signal Officer for 1890, appears to afford the means of 

 more clearly expressing the relations that exist between the mass 

 of the atmosphere and the forces available for the generation 

 and movement of storms. Its tentative application at the Signal 

 Office has anticipated and explained storm movements not in- 

 dicated or accounted for by the usual methods. 



As pertinent to this matter, there is instanced a study of the 

 progress of thunder-storms made by Berg, who observes that the 

 line of storm front in every case inA'estigated made a decidedly 

 conspicuous bend into the densest part of the lines representing 

 the absolute humidity. 



Scientific conditions have so changed that in these later years 

 it becomes more and more difficult for investigators to publish 

 any work which may he characterized as magnum opuv. Under 

 this head, however, must be classed Buchan's important memoir 

 on the distribution of atmospheric pressure, temperature, and 

 wind direction over the whole world; a large quarto volume, which 

 contains much new material. It has been incorporated with 

 the results of observations during the Challenger expedition, in 

 which series this work appears. The isobars and isotherms for 

 each month in the year for the whole earth are charted on 

 Mercator's projection, and for the northern hemisphere on a 

 chart constructed on a polar projection. In connection with 

 an abstruse subject, to which Buchan has paid so much atten- 

 tion, the diurnal variation of pressure, he opines from the Chal- 

 lenger observations that the oscillations are due to the heat taken 

 from the solar rays directly in passing through the air and in- 

 stantaneously communicated through the Avhole mass from top 

 to bottom by heating and evaporation of water on innumerable 

 dust particles. The afternoon minimum, he thinks, is caused by 

 upward currents removing a portion of the lower air. Marked 



