112 MOTIONS AND PEESSUEE OF THE ATMOSPHEEE. 



force now than when it was first made ; because, since that period, the 

 sailor has greatly extended the use of the Aneroid Barometer, which, by 

 its immediately showing any atmospheric change, gives a longer and more 

 assured warning. The Mercurial Barometer used at sea, from having its 

 column much contracted in one part to overcome the "pumping" action 

 which the vessel's motion would cause, is much more sluggish in its action, 

 and hence it may not indicate a change till a part of its predictions has 

 passed over. 



(30.) But the isolated observation of the Barometer, and other indica- 

 tions of the condition of the atmosphere, made at sea, must necessarily be 

 of infinitely less value in guiding the sailor than are those made within the 

 range of the stations from which telegraphic Weather Eeports are sent to 

 each other, and by a comparison of which many very important conclu- 

 sions can be quickly arrived at. It is this minute examination and com- 

 parison of the varied state of the meteorological conditions existing at one 

 and the same moment at stations widely separated and extending over 

 the West and N.W. of Europe, that some remarkable laws have been 

 arrived at, and great advances made in the general subject of atmospheric 



meteorology. 



(31.) But, although this method of comparison is not possible to the 

 ship at sea, yet it is thought that the method by which "weather pre- 

 dictions" are made; the reasons why barometric pressures indicate certain 

 general facts; why land disturbances from heat and cold, and differing 

 temperatures in sea water, bring about, or point to, great and broadly- 

 marked differences in wind and weather ; are proper subjects for the sailor's 

 study, and, as such, are not out of place here. 



In laying as briefly as we can the results arrived at by the various ob- 

 servers, or by a combination of their deductions, we shall mostly give the 

 words of the authors themselves, referring to the works quoted for more 

 extended information. 



(32.) When the Cyclone theory was propounded, which was in the first 

 place done by Mr. Eedfield, then by Sir W. Eeid, Mr. Piddington, Dr. Thorn, 

 and other pioneers in this all-important subject, the " Law " was established 

 that these Hurricanes in the Northern Hemisphere, having a low barometer 

 in the centre of their vortex, will have this centre or vortex on the left hand 

 of any one standing on its circumference with his hack to the wind ; the case 

 being reversed in the Southern Hemisphere. 



In the further investigation of the laws which govern the movements of 

 the air, consequent on the recommendations of the Brussels Congress, the 

 Dutch nation, well known to be foremost in the rank of enquirers, in 1857 

 established an Observatory at Utrecht, known as the Eoyal Meteorological 

 Institute of Holland ; and here the late Dr. Christopher H. D. Buys-Ballot 

 pursued for several years an enquiry into the data afforded by the daily 

 weather telegrams which were sent to this Observatory from several parts 

 of the kingdom, and he arrived at the very important conclusion that the 

 same Law which had been established by Eedfield and Eeid in regard to 

 the local meteors, known as Cyclones, is applicable, on the very much 

 broader scale, to the whole system of winds over the whole of the globe. 

 But confining the remarks to the Northern Hemisphere, and more parii- 



