354 ON THE METEOROLOGY OF IRELAND. 



then greatly influenced by local causes. Accordingly, excluding 

 those cases in which the wind did not exceed a light breeze at most 

 of the stations, the remainder were examined in detail, by laying 

 down the simultaneous directions of the wind upon a series of 

 skeleton charts prepared for the purpose ; and there was no diffi- 

 culty in ascertaining, by the inspection of these charts, the exist- 

 ence or non-existence of rotatory movement. The Fame means 

 sufficed to determine, very nearly, the position of the centre of the 

 vortex at each epoch ; and the places of the centre being thus 

 found, for epochs distant by intervals of twelve hours, the direction 

 and velocity of its progressive movement are ascertained. 



The position of the centre of the vortex at any instant may be 

 determined, more accurately, by calculation. Thus, if y and x 

 denote the distances (in geographical miles) of the place of obser- 

 vation from any assumed central point, measured on the meridian, 

 and on the perpendicular to the meridian, respectively ; ?/ and ^ 

 the corresponding co-ordinates of the centre of the vortex ; and 

 the angle which the direction of the wind at the point (y, x) makes 

 with the meridian, measuring from north to east : 



y - y + (x - a*,) tan = 0, 



the direction of the wind being perpendicular to the line connect- 

 ing the points (y, x) and (y , x ). Now, all the quantities in this 

 equation are given, excepting y n and as a ; so that, if the direction 

 of the wind be accurately known at two stations, the co-ordinates 

 of the centre of the vortex may be completely determined. The 

 irregularities due to local causes, and the errors of observation 

 themselves, forbid this ; and, in order to lessen their influence, it is 

 necessary to know the direction of the wind at several stations. 

 There will then be as many equations of the preceding form as 

 there are places of observation ; and the unknown quantities, ?/ 

 and ar , are to be determined by combining these equations by the 

 method of least squares. 



It is found, that the centre of the vortex is also the point of 

 least barometric pressure, and that the pressure increases regularly 

 with the distance from it. Hence the position of the centre may 

 be inferred from the barometric observations alone. The positions 

 thus determined have been found to coincide in all cases, very 

 nearly, with those deduced from the observed directions of the 

 wind. 



