1877.] 



the Lines of equal Barometric Pressure. 



527 



differences is tliat for May, which also depends on small negative values 

 of /3, and /S^, yet gives a result differing but little from the mean, 

 though the direction of the wind (a little to west of north) is upwards 

 of 100° from the mean direction *. 



Relation of the difference of barometric pressures for 100 miles to the re- 

 sultant force of the wind, — The differences of barometric pressure for 

 100 geographical miles perpendicular to the isobars (100 mile gradients t) 

 have been calculated from the values of /S^, ,82, and ; these, with the 

 resultant mean pressures for the wind obtained as indicated p. 525, are 

 given in Table YII. The 6th column contains the mean pressure (P) of the 

 wind in pounds on the square foot of surface, derived from the results for 

 the three stations. The last column contains the differences of bard- 

 metric pressure for 64 miles of interval, for which, in the mean, one 

 thousandth of an inch of barometric pressure is equivalent to one 

 hundredth of a pound of pressure of the wind. 



Table VII. — Eesultant forces of the Wind and difference of 

 Barometric Pressure, 1843-1846. 







Eesultant force of wind. 





Month. 



/3 for 

 100 miles 











/3, for 

 64 miles. 







Green. 



Mak. 



Dubl. 



Mean 

 P. 







in. 



lb. 



lb. 



lb. 



lb. ■ 



in. 





0074 



0-66 



0-49 



0-71 



0-62 



0047 





•035 



•22 



•20 



•14 



•19 



•022 





•035 



•21 



•27 



•28 



•25 



•022 





•028 



•34 



•11 



•20 



•22 



•018 



May 



•016 



•23 



•07 



•09 



•13 



•010 





•036 



•27 



•25 



•17 



•23 



•023 





•046 



•29 



•23 



•20 



•24 



•028 





•040 



•25 



•12 



•14 



•17 



•026 





•017 



•08 



•08 



■04 



•07 



•Oil 





•057 



•41 



•26 



•32 



•33 



•036 





•054 



•40 



•29 



•23 



•31 



•035 





•040 



•16 



•37 



•26 



•26 



•026 





036 



0^25 



0-20 



0-23 



0-23 



0^023 



* This agreement is probably to some extent accidental : it will be seen that the 

 direction of the wind at Dublin was —27° (27° west of north), while it was near 

 north at Greenwich, and 24° east of north at Makerstoun. When the resultant 

 direction for May at Dublin is determined by the frequency of the wind from different 

 points (by Lambert's formula), it is found to be 4-20°, nearly as at Makerstoun ; the 

 difference, — 0, for May would then become -}-6°. I may remark that the resultant 

 directions for the other months at Dublin obtained by the same formula agree better 

 with the mean of those at Greenwich and Makerstoun than when the "runs" of the 

 pencil are employed. The resultant direction for the four years is 256° by the 

 frequency, or 5° further south of west than when determined by the pencil-runs. 

 The resultant direction at Makerstoun for the four years is exactly the same by the 

 two methods. 



t I have felt some difficulty in emploj-ing the word gradient, due to Mr. Thomas 

 Stevenson, C.E., in this relation, as it is associated to some extent with a hypothesis of 



