286 LIGHT SCIENCE FOR LEISURE HOURS. 



present difficulties on a close investigation which com- 

 pel us at once to recognise 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 phenome- 

 non as a part merely as indeed it is of the marvel- 

 lously complex circulation 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 elucida- 

 tion 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. Eoss'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 : 



