The Rev. H. Lloyd on the Meteovolojy of Ireland. 449 



simultaneous directions of the wind upon a series of skeleton charts prepared 

 for the purpose ; and there was no difficidty in ascertaining, by the inspection of 

 these charts, the existence or non-existence of rotatory movement. The same 

 means sufficed to determine, very nearly, the position of the centre of the vor- 

 tex 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 progres- 

 sive 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 observation from any assumed central 

 point, measured on the meridian, and on the perpendicular to the meridian, 

 respectively; ya and ^o the corresponding co-ordinates of the centre of the vor- 

 tex ; and the angle which the direction of the wind at the point (y, .r) makes 

 witli the meridian, measuring from north to east ; 



y - yo+ (•«■■ - *o) tan = 0, 

 the direction of the wind being perpendicular to the line connecting the points 

 (y, x) and (yo, ^o)- Now, all the quantities in this equation are given, ex- 

 cepting ?/o and Xo ; 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 obser- 

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

 cessary 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, y„ and x^, 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 observa- 

 tions alone. The positions thus determined have been foimd to coincide in all 

 cases, very nearly, with those deduced from the observed directions of the wind. 



The following are the well-marked instances of aerial rotation which have 

 occurred in Ireland in the course of these observations. No case has been in- 

 cluded in the enumeration, in which the simultaneous directions of the wind 

 did not differ, at two points, by at least 90"; and thus, probably, many cases 



