1877.] 



the Lines of equal Barometric Pressure. 



537 



fact, that if a mass of air changes its latitude, from whatever cause, it 

 will not proceed in the direction in which it was originally propelled or 

 drawn ; but if it passes to a higher latitude the motion will be towards a 

 point more easterly, and if to a lower latitude towards a point more 

 westerly than the original direction ; that is, in our hemisphere the ulti- 

 mate dh'ection of motion will be positive of the original direction. 



The friction of the moving mass of air against the earth's surface is 

 always recognized as a cause which diminishes the amount of this dis- 

 placement. Thus Hadley, on the lower currents from the tropics towards 

 the equator : — " Before the air from the tropics can arrive at the equator, 

 it must have gained some motion eastwards from the surface of the earth 

 or sea whereby its relative motion will be diminished"*. The upper 

 current moving towards the north is supposed by him to preserve its excess 

 of velocity till it descends. So Sir John Herschel on the same hypo- 

 thetical upper current : — " In flowing over to regain its level, it com- 

 mences its course relatively in a meridional direction, but really with the 

 full amount of easterly velocity which the earth's equator has ; and since 

 this, as it proceeds north or southwards, is in excess of what would suffice 

 to keep it on the same meridian, it continually deviates to the westward 

 [i. e. the direction from which it appears to proceed] ; and when it again 

 returns to the earth in its circulation, which it does on both sides beyond 

 the tropics, it does so with a powerful westward tendency, and the more, 

 as in its course it has been less under the injliience of surface friction owing to 

 the elevated region in which it has travelled" f. I have put in italics the 

 explanation of the difference of directions of the currents found by me. 



The theory of the trade-winds is so mixed up with unknown currents, 

 ascending and descending, and presents so many other difficulties as 

 usually stated, that it is not easy to determine what may be due to differ- 

 ences of latitude velocit}^, and what to other causes in the production of 

 winds with definite directions in different parts of the globe. It is not 

 a little remarkable, however, that no writer, as far as I am aware, had 

 perceived that if the difference of latitude "\-elocity affected the direction 

 of motion of a vertical mass of air, this fact should be observed at once, 

 especially in middle latitudes, in the movements of the wind and of the 

 clouds. 



If, as Sir John Herschel shows, the lower stratum of a mass of air 

 moving northwards has its tendency to move eastwards most diminished, 

 this diminution will be communicated gradually by the viscosity of the air 

 from layer to layer till the upper stratum of the mass in motion is 

 attained where "the influence of the surface-friction" is leastj. AYe 



* Pliil. Trans. 1735, p. 61. 



t Meteorolog}', by Sir J. Herschel, 1861, p. 59. 



\ It is always understood that friction diminishes the velocity in all directions ; but 

 the cause which propels or draws a mass of air from one latitude to another is pro- 

 bably continuous for some time at least. 



