396 



Second Report of the Meteorological Committee of [April 



The observations of Mr. Mc Hardy, though extending only to latitudes 

 south of the equator, and, though evidently njade with far less care, and 

 with an instrument in which the fluctuations arising from the motion of 

 the ship are very imperfectly destroyed, yet, when reduced and grouped 

 in a similar manner, afford a result agreeing in their general tenor very 

 satisfactorily with those of Sir E.Ryan. To render them comparable, 

 as the zero of Mr. McHardy's barometer is unknown, a correction of—* 

 0.188 has been applied to all his reduced observations, by which the 

 equatorial indications of the two barometers are made to agree, and the 

 following table exhibits their results when so reduced, grouped, and 

 corrected : — 



Limits of the Zone of Latitude. 



Number of 

 Days' Observa- 

 tions. 



Mean Pressure 

 in Inches. 



Mean correspond- 

 ing Latitude. 



Lat. 0° N. to 5° S. 



8 



29 821 



1° 42' 



_ 5 — to 15 — 



5 



29.802 



9 20 



— 15 — to 25 — 



6 



29.960 



19 41 



25 — to 35 — 



16 



30.085 



31 20 



The total depression concluded from the latter series of observations 

 agrees very nearly in amount with that stated by Sir J. Herschel, as the 

 result of his own observations during itis voyage from England. The 

 general fact may now therefore be looked upon as unequivocally esta- 

 blished, and it is hoped that it will henceforth attract the attention of 

 all voyagers ; and that observations will be diligently accumulated for 

 the purpose of ascertaining the law of variation of atmospheric pres- 

 sure in all latitudes both within and beyond the tropics, and in either 

 hemisphere, since it is very possible that the same exact law may 

 not be found to apply to both, and that the Atlantic, Indian, and 

 Pacific Oceans may offer differences depending on their different extent 

 and relation to the continents adjacent to them. 



If, in a report like this, it be allowed to speculate on the causes of 

 meteorological phenomena, it appears extremely probable that the 

 equatorial depression in question arises from the same cause which 

 produces the trade winds, viz. the rarefaction and consequent ascent of 

 the equatorial air, which, although constantly supplied from the extra- 

 tropical latitudes, is yet not supplied instanler, nor without a due 

 dynamical motive force, which, in a free elastic fluid, can be no other 

 than an excess of pressure on the side from which the supply is drawn, 

 or (which comes to the same thing) a diminution of it, in the nature of 

 a " suction," on that side towards which the superficial currents rush ; 

 which excess and diminution obviously arise from the overflow of the 

 unsustained portion of atmosphere above the equatorial zone into the 

 regions beyond. The inquiry, therefore, connecting itself as it does, 



