524 



Mr. J. A. Broun on the Directions ^c. of [Feb. \, 



Directions and Force of the Wind. — In most meteorological investiga- 

 tions, and especially in those connected with climate, it is essential to 

 obtain the force and frequency of the winds for each point of the com- 

 pass. It has been usual, however, in many researches to determine the 

 resultant direction and force for given periods by the known law of the 

 composition of forces. It is quite possible that this resultant may be in 

 a direction from which no wind ever blows, but it does not the less re- 

 present what all the other winds would have been equivalent to, both in 

 direction and force. It may also be affirmed that in fact the resultant 

 direction thus obtained is generally that of the prevailing wind. 



In questions such as the present, if any general relation subsists 

 between the direction of the isobars and that of the winds, the 

 mean direction of the one will probably be affected in like manner 

 with that of the other*. It occurred to me then to compare the 

 resultant directions of the wind with those of the isobars for each month 

 of the year. Possessing these resultants for Makerstoun for each month 

 derived from four years' observations (1843-1846) f, it was necessary to 

 compute the corresponding values for Greenwich and Dublin, and to 

 calculate the mean directions and intervals of the isobars for the same 

 years. 



The observations at Greenwich were made with an Osier's anemo- 

 meter, and only pressures which on the mean for an hour exceeded one 

 fourth of a pound were noted. At Makerstoun the instrument employed 

 was of a wholly different description and not self -registering ; observa- 

 tions were made of the maximum pressures (if not less than one tenth 

 of a pound) within ten minutes at each observation hour (see the In- 

 troductions to the Makerstoun Observations). Observations of the direc- 

 tion and force of the wind were not begun at the observatory of Trinity 

 College, Dublin, till 1845 ; but I have obtained observations for the four 

 years in question from the volume of observatious made at the Ordnance 

 Observatory, Phoenix Park, in a locality stated by Dr. Lloyd to be wholly 

 unexceptionable^. The instrument employed at the Phoenix Park obser- 

 vatory was a Whewell's anemometer, which always worked imperfectly, 

 so that the observed runs of the pencil differed from time to time for 

 the same wind-velocities, §. The monthly resultants calculated from the 



It is to be observed that the mode in which the resultant wind-direction is 

 obtained is not the same as that employed for the direction of the lines of equal 

 pressure ; but this difference can only alfect results derived from periods during which 

 the wind has blown frequently in very different or opposite directions ; and even in 

 such cases the isobars should retain their relation to the predominating wind, if the 

 resultant force for that direction is considerable. A special investigation will show, 

 however, the amount of error which may be diie to this cause, 

 t Trans. Eoy. Soc. Edinb. vol. xix, pt. 2, p. 99, 



X Observations made at the Observatory, Trinity College, Dublin, vol. i. p. 384. 

 § Introduction to Observations made at the Phoenix Park Observatory. 



