METEOROLOGY. 505 



would be experienced by dry air. From these two principles, which 

 were established, the former by Dalton and the latter by Joule and Sir 

 William Thomson, it follows that the pressure of vapor in the air and 

 its condensation exercise a powerful influence in diminishing the press- 

 ure." In this sentence the two principles are correct, and due to the 

 physicists named, but the conclusion is Buchan's, and, as stated, does 

 not follow from these principles, but from others, about which he is 

 silent. 



[If there were no vapor at all in the air, and were the seas replaced by 

 polished silver and the continents by dry rock, we should still have 

 a similar general distribution of pressure, due then, as now, not to vapor 

 in and of itself, but to the winds that will themselves be produced by 

 unequal distribution of temperature and density. That the condensa- 

 tion of vapor to fog and rain does not directly produce a diminution 

 of barometric pressure has been thoroughly demonstrated'. On the 

 contrary, the latent heat evolved by condensation of vapor expands 

 the air so much that a decided increase of pressure should result. The 

 simple truth is that ascending currents must be followed by inflowing 

 descending and horizontal currents to fill the vacancy. These soon 

 set up a whirl, and the barometer falls as a result of the centrifugal 

 forces developed by the two motions, i. e., about the earth's axis and 

 about the center of low pressure. It is therefore very improper to say 

 that the air flows inward because of the great observed barometric 

 depression, or that the presence or the condensation of the aqueous 

 vapor causes the depression.] 



[Buys Ballot's law of wind and pressure, as worded by Buchan, is the 

 expression of the concordance of two results, and is not an expression 

 of physical laws connecting cause and effect. It is a rule, not a law. 

 The frequent mention of Buys Ballot's law by Buchan and others, 

 especially English writers, long since led the present writer to look up 

 the history and bibliography of this rule, and it will perhaps be a mat- 

 ter of surprise to many to find that Buys Ballot himself never pub- 

 lished or claimed it in anything like its present form. It would be 

 much more proper to attribute the law as first enunciated by Buchan 

 in 1866 to Buchan himself or to the cyclonologists Keid, Piddington, 

 Bedfield, &c, and attach Buys Ballot's name merely to his own rule, 

 namely: "In Holland, when on any day the barometric departures 

 from normal values indicate a gradient between any two stations, then 

 within the next twenty-four hours the wind will blow nearly at right 

 angles to that gradient, and irom left to right if one's face be towards 

 the lower barometric reading."] 



In his section on the connection between steepness of barometric 

 gradient and velocity of attending windb, Buchan gives recent results 

 for 8 A. M., deduced by Whipple from the continuous auemometric 

 records at Kew Observatory for 1875 to 1879, inclusive, as compared 



