116 MOTIONS AND PEESSURE OF THE ATMOSPHEEE. 



continents and islands. Over Siberia, in January, the mean height of the 

 barometer is nearly thirty-one inches, whilst South of Iceland and Green- 

 land it is about twenty-nine and a half inches, which fact well accounts 

 for our prevaiHng S.W. winds in winter. 



" Knowing these facts, it is easy to realise that the storm-centres whiclr 

 come from the Atlantic towards our West coasts pass along the Northern 

 side of the large area of high barometer which prevails in the neighbour- 

 hood of the Azores, and have the area of low barometer which prevails to 

 the Southward of Iceland to the Northward of them. They are therefore, 

 as it were, subsidiary to the larger system which is situated in the Atlantic, 

 more especially in winter, and their Westerly winds are generally stronger 

 than their Easterly winds, owing to the fact that the greater barometer 

 difference or gradient is on their Southern sides." 



(36.) BAK0METRIC Waves. — In 1843, the late Sir John Hersch el presented 

 a report to the British Association on the result of some investigations he 

 had made into the barometric changes registered at the term-days of the 

 Equinoxes and Solstices, and arrived at the following conclusions : — 



"1. We have succeeded in tracing distinct barometric waves of many 

 hundreds of miles in breadth over the whole extent of Europe. Not only 

 the breadth, but the direction of the front and the velocity of the progress 

 of such waves have been clearly made out. 



"2. Besides these distinctly terminated waves, we have been able, if 

 not to trace the rate and law of progress, at least to render very evident the 

 existence of undulating movements of much greater amplitude, so great 

 indeed as far to exceed in dimension the area in question, and to require 

 much more time than the duration of a term-series (36 hours) for their 

 passage over a given locality. At the same time it must be recollected that 

 the records of every meteorologist bear ample testimony to this conclusion 

 in the fact of long-continued rises, falls, and stations (both high and low) 

 of the barometer continuing for many days or even weeks. 



"3. In Europe, Brussels is clearly entitled to be regarded as a point of 

 comparatively gentle barometrical disturbance. Very deep waves it is true, 

 and very extensive ones ride over it ; but with regard to smaller ones it 

 may be regarded as in a certain sense a nodal point where irregularities are 

 smoothed down, and oscillatory movement in general is more or less 

 checked ; and such movements increase in amount as we recede from 

 Brussels as a centre, especially towards the N.W. as far as Markree." 



(37.) Mr. W. R. Birt, in the further enquiry into the subject of barometric 

 waves, as reported to the British Association, between 1843 and 1848, 

 estabUshed in the year 1846 the fact that each wave is accompanied by two 

 streams of air, constituting two oppositely directed winds ; and it is not 

 a httle interesting to remark, in passing, that this discussion, although 

 unaccompanied by the actual projection either of barometric gradients or 

 isobars, anticipated them both, inasmuch as the heights of the barometer 

 are so arranged as to serve all the purposes of projected isobars and the 

 determination of gradients, and this was effected as early as 1844, by 

 barometric heights being treated geographically — an essential element of 

 the isobar. 



It will be needless to follow the progress of this subject through all its 



