The Loiu Barometer* of the Antarctic Temperate Zone. 335 



cause, or which we can explain on simple principles. Even 

 those marked phenomena which appear most easily referable 

 to simple agencies, present difficulties on a close investigation, 

 which compel us at once to recognize the efficiency of more 

 causes than one. For instance, the phenomenon of the trade- 

 winds, as explained by Halley, appears at first sight easily 

 intelligible ; but when we look on this phenomenon as a part 

 merely — as indeed it is — of the marvellously complex circu- 

 lation of the earth's atmosphere — when we come to inquire 

 why these winds blow so many days in one latitude, and so 

 many in another, or why they do not blow continually in any 

 latitude — when we consider the character of these winds as 

 respects moisture and temperature, the variation of the velocity 

 with which they blow, and of the quantity of air they transfer from 

 latitude to latitude — we encounter difficulties which require for 

 their elucidation the comparison of thousands of observations, 

 or which baffle all attempts at elucidation. 



There is, however, one atmospheric phenomenon — that 

 which I have selected for the subject of this paper — which 

 presents a grand simplicity, rendering the attempt at a simple 

 solution somewhat more hopeful than is usually the case with 

 meteorological phenomena. The discovery of this phenomenon 

 formed one of the most interesting results of Captain Sir J. C. 

 Ross's celebrated expedition to the Antarctic Ocean. He 

 found, as the result of observations conducted during three 

 years, that the mean barometric pressure varied in the following 

 manner at the latitudes and places specified : — 



South latitude. Height of the barometer. Place. 



0° 



0' 



29-974 in. 



At sea. 



13 







30-016 



— 



22 



17 



30-085 



— 



34 



48 



30-023 



Cape of Good Hope & Sydney, 



42 



53 



29-950 



Tasmania. 



45 







29-664 



At sea. 



49 



8 



29-469 



Kerguelen & Auckland Isles. 



51 



33 



29-497 



Falkland Isles. 



54 



26 



29-347 



At sea. 



55 



52 



29-360 



Cape Horn. 



60 







29-114 



At sea. 



66 







29-078 



— 



74 







28-928 



— 



We see here a gradual increase of barometric pressure 

 from the equator to about 30° south latitude, and from this 

 point at first a gradual diminution — so that in about 40° south 

 latitude we find the same pressure as at the equator, and 

 thence a more rapid diminution. The rate of change is illus- 



