METEOROLOGY. 501 



Philosophical Transactions. In the present essay he gives his latest 

 views, devoting to the subject four pages or rather more space than 

 would have seemed appropriate had he not paid such special attention 

 to this subject. In the main, his argument is that the warming and ex- 

 pansion of the atmosphere by the sun causes a compression or tension that 

 increases until it overcomes the resistances due to the inertia and vis- 

 cosity of air, when the latter by its expansive movement experiences re- 

 lief and the tension diminishes until the accession of heat from all 

 sources ceases. Moist air absorbs heat and expands more than dry, there- 

 fore the diurnal fluctuation is greater over moist than over dry land, 

 and is largest within 10° of the equator ; the sea surface temperature 

 varies so little during the day that the diurnal fluctuation of pressure 

 over the ocean is not entirely due to that temperature but to direct 

 heating by absorption by the molecules of air and vapor. These ex- 

 pansions are followed by contractions at night, and as the air cannot 

 mechanically flow to and fro fast enough to fill the vacua, a consequent 

 diminution of tension is observed. On land the heated ground imparts 

 to the air a much larger diurnal variation of temperature, and by so 

 much increases the barometric fluctuation which, other things being 

 equal, is found to be greatest when the sky is clearest, i. <?., where the 

 most sunshine reaches the earth, and least when it is covered with 

 dense clouds and sunshine is cut off, being thus the reverse of what is 

 observed over the open sea. [Although nothing is said about the diur- 

 nal period due to the vapor thrown into the air by evaporation during 

 sunshine, and abstracted by nocturnal cooling, yet such would seem to 

 be equally important. The defects of this and all other similar theories, 

 of which there are many, have long since led the present writer to 

 abandon them, and in general adopt a view that he has frequently com- 

 municated to others and referred to in various publications, i. e., that 

 this periodicity in pressure is principally a dynamic phenomenon deduc- 

 ible from Ferrel's formula for general and special atmospheric move- 

 ments, whenever they shall have been satisfactorily developed into sine 

 and cosine series, with the time as the argument. A matter however 

 that at present offers more difficulties to the analyst than even the most 

 complex of astronomical theorems relative to the motions of the heavenly 

 bodies.] 



The Challenger cruise has also afforded Buchan excellent results as 

 to the diurnal variations in the force of the wind at sea. In columns 9 

 and 10 we give the force in Beaufort's scale numbers as read from 

 Buchan's illustrative wood-cut. The diurnal curve shows no distinct 

 uniform or reliable maximum or minimum on the open sea, but a marked 

 maximum at 1 or 2 p. M., when near land. The diurnal variation in 

 in the wind force or velocity thus depends largely on heating of earth 

 and water by direct solar radiation, and the explanation of Espy aud 

 Koppen is practically adopted by Buchan in saying that the ascension of 

 the air during the day thus brings down portions of the rapidly moving 



