190 BIRBS — PHYSICAL GEOGKAPHY. 



and western portion is also level, while the eastern portion is rolling, 

 and the southern becomes hilly as the Ohio River is approached. 



The average temperature throughout the year varies about five de- 

 grees; that of the vicinity of Cincinnati being 54° and of Northern 

 Ohio between 49° and 60° Fahrenheit. The difference between the tem- 

 perature of the winter is greater than that of the summer months. In 

 Northern Ohio the average temperature during winter is from 27° to 28°. 

 In Southern Ohio it is about 35°. In summer the average for Northern 

 Ohio is about 71°, for Southern Ohio 75°. 



There is a variation of about 14 inches in the mean annual rainfall 

 in dififerSnt parts of the State, that of the extreme south-west corner 

 being 46 inches and that of the Lake shore 32 inches. With the excep- 

 tion of a few localities, of limited extent, the decrease in rain-fall is 

 gradual, and about one inch for every twenty miles ascent in latitude, 

 and bears a direct ratio to the annual temperature. The prevailing 

 winds are from the west. 



Otherwise than above stated, the topography of the State is uniform. 

 No large bodies of water are found, the largest being artificial reservoirs 

 constructed in connection with the various canals for the purpose of 

 securing a uniform and regular supply of water for the same. The 

 largest of these is the St. Mary's reservoir, situated in Mercer and Au- 

 glaize counties, which covers an area of 17,000 acres, to a depth of ten 

 feet. Three others, of smaller dimensions, are the Lewistown, in Logan 

 county, Loramie, in Shelby, and Licking, in Licking, Fairfield, and 

 Perry counties, all, except the latter, which is nearly central, are in 

 Western Ohio. In the north-eastern portions of the State numerous 

 small lakes are situated. 



The rivers of the State are numerous and many of them are large. 

 Those emptying into the Ohio flow through the wide valleys formed by 

 the action of their own waters. These diverge somewhat in their course 

 to the Ohio. Those emptying into the Lake are fewer in number, and 

 converge towards the Lake. 



The Ohio River separates the southern, and more than half of the 

 eastern border of the State from the adjacent States, and is one of its 

 natural boundaries. In its course westward it forms a considerable 

 curve to the south, the lowest point of which is near the middle of the 

 southern border of the State. 



Lake Erie extends into the northern border of the State at its middle, 

 causing a curvature of that border corresponding with though less 

 well marked than the convexity formed by the Ohio. In the western 

 portion of the Lake are several islands, which have afforded breeding- 



