112 On Dove's Law of the 



Glancing over the mean values of the different elements, 

 as shown in Table IV. [vide Plates I. and II.), we perceive 

 at once that a maximum of the barometer is constantly ac- 

 companied or immediately followed by a minimum in tem- 

 perature of air and tension of vapour, as on the 5th, 13th, 

 and 21st, the mean directions of the wind being, for those 

 days respectively, S.W., S.W., and S.E. : — 



Bar. Ther. P. V. Wind. 



5th— 30-000 .. 60-5 .. 0-338 .. S.W. 



13th— 30-034 . . 52-5 . . 0-278 . . S.W. 



21st— 30-292 .. 52-4 .. 0-242 .. S.E. 



Further, we see that a minimum of the barometer coincides 

 with a maximum in temperature and aqueous pressure on the 

 3rd, 11th, 19th, 30th :— 



Bar." 



Ther. 



P. V. 



Wind. 



3rd— 29-809 . 



. 62-9 . 



. 0-279 . . 



n.w: 



11th— 29-555 . 



. 55-4 . 



. 0320 . . 



N.N.W. 



19th— 29-847 . 



. 59-5 . 



. 0286 . . 



N.E. 



30th— 29-772 . 



. 70-1 . 



. 0-324 . . 



N. 



In the first instance we have the polar current with its 

 properties, and in the last the equatorial ; the intermediate 

 days show signs of the combined action of both currents. 

 The pressure of dry air accommodates itself closely to the 

 pressure of air, as shown in the respective columns, and the 

 amount of cloud reached its maximum at the same time as 

 the barometer, and the rain fell abundantly when the tem- 

 perature was very low (52 - 0). 



If such diagrams and tables are calculated to illustrate the 

 simultaneous occurrences in the various meteorological ele- 

 ments, they must still fail to exhibit the law of alternating 

 currents as closely as we could wish ; another mode is there- 

 fore necessary. I refer to the construction of wind-roses, or 

 barometric, thermometric and atmic cards, the nature and con- 

 struction of which I shall endeavour to explain in as simple 

 a manner as possible. 



By taking the means of all the readings of the barometer, 

 thermometer, or hygeometer,when the wind blewfrom a certain 

 point of the compass, we obtain the mean reading of these me- 

 teorological elements for that point; and by taking the mean in 

 this way for each point of the compass, we obtain what is called 

 a barometric, thermometric or atmic wind-rose. ( Vide Plate 

 III.) But I must mention here, that we have first to eliminate 



