DYNAMICS OF CYCLONES AND ANTICYCLONES. 141 



and the depression soon get filled up. So the cyclone has to move on for fresh supplies, 

 and if our experimental illustration is correct, it will move along in the direction of 

 the strongest tangential winds. 



Now, if we look at the Synoptic Charts issued by the Meteorological Office, we 

 shall see that they support this explanation, that these tangential winds are the 

 principal cause of the movement of the cyclone as a whole, that the centre of depression 

 moves in the direction and nearly parallel to these winds ; or, in other words, the 

 cyclone moves nearly parallel with the isobars at the side on which the barometric 

 gradient is steepest. Further observation will show that the greater the difference 

 in the barometric gradients on the two sides, the quicker is the advance of the cyclone, 

 due to the tangential energy on the steep side being greater than on the other. The 

 examination of these charts will also show that, whenever the isobars surrounding the 

 cyclone become equally spaced on all sides, that is, when the barometric gradient is the 

 same all round the centre — or even on the front and sides — that the cyclone travels 

 very slowly and is generally in a dying condition. The air under these conditions comes 

 to the cyclone with equal velocities from all directions, and there is no tendency for 

 the centre to move. The tangential force is still in great abundance, the winds blowing 

 in from all directions systematically all round, but there is no unbalanced tangential 

 force, and the centre remains over the same area, and as the hot moist air — the source 

 of the energy — is soon used up, the winds tend to fall and the depression to fill up. 

 This seems to be the history of many of the cyclones with circular and concentrically 

 placed isobars, though in winter they seem occasionally to linger for a time, their winds 

 then seem to depend a good deal on the energy of surrounding anticyclones. It also 

 seems possible that the movements of such cyclones may be determined a good deal 

 by differences of temperature over the anticyclonic areas. 



In illustration of these points we shall now refer to some actual examples taken 

 from the Synoptic Charts issued weekly by the Meteorological Office, London. These 

 charts show the distribution of pressure and direction of winds over Europe from 

 observations taken at 8 a.m. and 6 p.m., the day interval being thus 10 hours, 

 whilst the night one is 14 hours. There are charts also giving the temperatures at 

 8 a.m. Plate I. is a reproduction of four of these charts, showing the distribution 

 of pressure over our area during the passage of two cyclones, — the one a typical 

 quickly-moving one, and the other a slowly-progressing one. In the charts for 10th 

 December 1898 we have the characteristic distribution of pressure, as shown by the 

 isobars, for a quickly-moving cyclone; whilst the charts for 9th December 1897 

 show the form of isobars associated with slowly-moving ones. In the 1898 cyclone 

 it will be observed that the barometric gradient to the south of the centre of the 

 cyclone was very steep in the 8 a.m. chart, as shown by the closeness of the isobars on 

 that side, whilst the gradients on the other side and in front were easy. The strong 

 winds, as the form of the isobars would lead us to expect, are all to the south of the 

 centre, and are blowing from the west, and the result was that the centre of this cyclone, 



