DISTRIBUTION OF RAINFALL. 263 



of these basins of supply ; the vapours from the Pacific are deposited 

 ■within a remarkably well-defined coast region between latitude 41° 

 and our boundary at the Straits of Juan de Fuca ; the rain pours 

 down with great intensity on tiie coast between the mouth of Columba 

 River and Cape Flattery. It is surprising how little rain falls on the 

 Pacific coast between San Diego and Gape Mendocino, and how 

 quickly the atmosphere becomes drained of its vapour as we leave 

 the coast and proceed inland in latitudes north of 41°. The coast 

 range of mountains here act powerfully as condensers by forcing the 

 air up their western slopes. 



" The densest part of the Gulf vapour is thrown over the delta of 

 the Mississippi River, and as far east as longitude 86" its axis of 

 diffusion can be traced distinctly to the west end of the Lake Erie : 

 it is inclined towards the northeast for two reasons — the effect of the 

 earth's rotation on a flow from the south, and the influence generally 

 of the prevailing westerly winds. A second sweep over the country 

 occurs in southern Florida, most likely due to the immediate proximity 

 of the Gulf Stream ; and there is a third, as yet undefined, influx, 

 passing through Georgia and South Carolina. 



" The condensation of vapor from the Atlantic is most apparent a 

 short distance inland at the following localities : Along the coast of 

 Maine near Eastport and near Portland, in central Connecticut, 

 western Massachusetts, and extending to southern Vermont, and 

 near the entrance of Chesapeake Bay. Upon the whole, hills and 

 mountain ranges appear to have a comparatively small directive 

 influence upon the distribution of the rain. Florida, which may be 

 considered as almost perfectly flat, exhibits well-defined bounding 

 lines of rain distribution. River courses also seem to influence the 

 amount of rain, as along the Rio Grande. At the mouth of the Hudson 

 River, the curves become suddenly contracted ; and some similar 

 feature can perhaps be traced out on the Mississippi delta near New 

 Orleans. Beyond furnishing by their evaporation a supply to the 

 general fund of moisture, the Great Lakes do not appear to exercise 

 any direct influence ; on the yearly average the rains along their 

 borders are not increased. There is even a remarkably small amount 

 of rain-fall in northern New York, close to Lake Ontario. The efiect 

 of equalizing the temperature produced by all large bodies of water 

 has no doubt a direct influence upon the distribution of rain ; the 

 greater and more sudden the variations in temperature, the greater, 

 comparatively, the rain-fall. 



" The laws of the distribution of the rain-fall, as far as they depend 



