NOTES OF A NATURALIST. 



constantly modified by the changing position of the 

 earth in her orbit, the element of season is primarily 

 to be considered. It being admitted that the origin 

 of these disturbances is to be sought in the abnormal 

 heating or cooling of some considerable portion of 

 the earth's surface, it would seem that, in the case 

 of the Atlantic, local causes can have little effect, 

 unless we suppose that the heating of the surface 

 of the Azores in summer, or the annual descent of 

 icebergs from the polar seas, are adequate to influence 

 the march of a travelling cyclone. 



On the evening of the 20th the barometer 

 had risen again to its former position, rather over 

 39'40 inches ; the mean of the four following days was 

 30'55, and that of the entire run from Southampton 

 to Barbadoes was 30'36. This fact of the continuance 

 of high or low pressures at the sea-level at certain 

 seasons in some parts of the world has scarcely been 

 sufficiently noted in connection with the ordinary 

 rules for the measurement of heights by means of 

 the barometer. The tables supplied to travellers are 

 all calculated on the assumption that the pressure at 

 the sea-level is constant — the English tables fixing 

 the amount at 30'00 inches of mercury, those calcu- 

 lated on the continent starting from a pressure of 

 760 millimetres, or about 29-92 1 inches. It is ad- 

 mitted that this mode of determining heights, when 

 comparative observations at a known station are not 

 available, is subject to serious unavoidable error. With 

 regard, however, to mountains not remote from the 

 sea-coast, it may be possible to lessen this incon- 

 venience in many parts of the world by substituting 



