1877.] the Lines of equal Barometric Pressure. 



525 



numbers of runs hare been combined by me in the same way as for the 

 forces to obtain the means for the four years. 



The resultant forces cannot therefore be considered absohite measures 

 at any of the three stations ; I have, however, endeavoured to reduce 

 them to a common unit*. 



In order to determine the factor required to give approximately the 

 true ^ind-pressures, I have assumed that the mean velocities of the 

 \^dnd at Grreenwich and Makerstoun do not differ much from those 

 shown by Eobinson's anemometers at Kew and Oxford. Mr. Johnson 

 found that the mean velocities at Keru^ and Oxford were nearly the 

 same t ; the mean velocity at Oxford, 110 feet above the ground, from 

 four years' observations (1857-61) was 10*2 miles an hour. This velocity, 



employing the generally accepted formula P = ^QQjgi'^es a'constant mean 



pressure of 0-52 lb. on the square foot of surface % ; the mean of the 

 observed pressures duriug the four years 18-13 to 1846 (without re- 

 ference to direction) was 0*46 lb. at Makerstoun and 0'48 lb. at 

 Greenwich, an agreement which, considering all the circumstances, the 

 difference of instruments, modes of observation, and distances from 

 the ground of the air stratum whose force was measured, must be quite 

 accidental. 



The resultant forces for Makerstoun have then been multiplied by 

 1-13 and those for Grreenwich by 1'08 to reduce to the mean, 0*52 lb. at 

 Oxford. In the case of the pencil-runs on Whewell's anemometer at 

 Dublin they have been divided by 50, which gives an approximation to 

 the mean pressures at the other stations. The means of the three 

 resultant pressures found for each month from the four years' observa- 

 tions may then be taken as approximations to the resultant mean 

 pressure for the mean resultant direction over the space included by the 

 three stations. 



The following Table contains the mean barometric pressure at Grreen- 

 wich, ^ith the differences (/3^ and /B^) from the mean pressures at 

 Makerstoun and Dublin (at mean sea-level), and the resultant direc- 

 tions of the wind at the three stations. 



* At Dublin and Makerstoun the vanes were nearly 20 feet above the ground (at 

 Greenwich. I believe tbe height was greater). I do not know any objection to the 

 position of the vanes which could vitiate the observed directions of the wind at any of 

 the stations. From my knowledge of the instrument used at Makerstoun and of the 

 care with which the observations were made, I believe the means to be very good 

 relative measures of the forces. 



t Eadcliffe Observatory Observations, vol. sviii., introduction to Meteorological Ob- 

 servations in 1857, p. xxiv. 



I This formula is, I believe, very far from being accurate ; the true relation between 

 the pressure and velocity of the wind, as shown by different anemometers, requires an 

 integrating pressure instrument, in which the sums of pressures may be shown by 

 work done. 



