1876.] 



of the Wind and Barometric Pressure. 



405 



curves, implying that the inferences drawn from the Bombay observa- 

 tion, which led to the construction of the former curve, are essentially 

 true. 



The evidence which the Bermuda wind results afford, taken together 

 with that which I have previously advanced, is, I think, sufficient to war- 

 rant the conclusion that the existence of such a system of diurnal wind- 

 currents should no longer be regarded as a working hypothesis merely, 

 but that it may indeed fairly claim to be regarded as an observed fact. 

 Accepting this conclusion, I proceed to show, in greater detail than was 

 justifiable from the evidence advanced in my former paper, what kind of 

 barometric variation may be expected to result from such a system of 

 wind-currents. Por this purpose the only results that will be used are 

 the mean diurnal variations of the north and east components of the wind 

 for the whole year at Bermuda. Strictly speaking, similar results for at 

 least one other station in a different latitude, but otherwise similarly 

 situated, are essential to the completeness of the explanation ; but, as 

 suitable observations for such a station are not yet available, their place 

 will be supplied by the simplest suppositions that can be made. The 

 results, however, will be seen not to depend wholly on those suppositions, 

 but it will be evident that similar conclusions will follow without 

 them. 



Bermuda, being a small island in mid-ocean, is specially favourably 

 situated for the investigation of such movements of the atmosphere as 

 form part of a general system of diurnal wind-currents affecting the 

 whole surface of the earth ; for in this case the investigation is not com- 

 plicated by the great difficulty of having to eliminate those peculiarly 

 local winds, such as the land- and sea-breezes, which are always found 

 on the coasts of extensive tracts of land, and such as the secondary 

 systems of diurnal wind-currents, of which we have indications in the 

 hot winds that blow in the daytime from the interior of large continents 

 like India, and which are to be attributed to local causes similar in cha- 

 racter to those M^hich produce the primary system of diurnal wind- 

 currents over the whole of the earth's surface — these secondary systems 

 originating in outward movements from the middle of heated continents, 

 in the same way that the primary system originates in outward move- 

 ments from the middle of the heated hemisphere. 



The quantity of air (measured by the number of square miles of sur- 

 face on which it rests) which enters or leaves the meteorological blockade 

 formed by the two contiguous full-hour meridian lines h—1 and h, and 

 by any two parallels of latitude d and &, may, with sufficient approxi- 

 mation, be calculated by the following formulae, provided that Q and b' 

 do not differ very largely : — 



AAA 



(1) 



