1886.] ON YAEIATIONS OF CLIMATE IN THE COURSE OE TIME. 19 



at all times suhjected to periodical cJianges, the duration of which 

 may he measured in thousands of years, and ivhich act in the 

 same direction within the same climatic area, and ivhich for one 

 period, are not great, hut ivhich. as alternations of strata are 

 often remarkahly regular^ seem to return after the lapse of a 

 fixed cycle of years. 



It is obvious tbat periodical changes in the streugth of the 

 Ocean cur rents ^yill cause correspondiiig changes in the climate 

 of the adjacent continents. Thus, if. for instance. the warm 

 North-Atlantic current, to \Yhich North Enrope owes the climate 

 which is so mild compared to its latitude, should increase in 

 strength, the climate there wonld doubtless become still milder. 

 Our shell-banks show that such changes in the temperature of 

 the sea have accompanied the climatic variatious. We are. 

 therefore, compelled to ask, what is the force which causes this 

 warm sea-current to flo w north\vards, and if we may assume 

 that there is some natural cause elfecting periodical changes in 

 the intensity of this force? The question being one as to a 

 climatic period, we must examine the great laws which govern 

 the climate. We must, of course, leave all temporary distui^- 

 bances of the air out of consideration, and only pay attention 

 to the great and simple laws which are revealed by the synoptic 

 charts of the average distribution of the aerial pressure at 

 various seasons. These charts show us : in the summer a low 

 pressure over the heated continents, but generally a higher one 

 over the cool oceans, and in the winter a higher pressure over 

 the cold continents, and a lower one over the oceans which 

 are warmer. 



In order to understand this varied distribution of pressure, 

 we shall imagiue an atmosphere, which everywhere has the same 

 degree of heat and the same height. The warmer the air. the 

 more it expands, so that the height of the atmosphere will 

 change if the temperature rises or falls. If we further assume 

 that the air cools or becomes quicker heated in s .me places 

 than others, the equilibrium will be disturbed. Over cold areas 

 the height of the atmosphere will decrease. The surface of the 



