190 BIRDS—PHYSICAL GEOGRAPHY. 
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 50° Fahrenheit. The difference between the tem- 
perature of the winter is greater than that of the summer months. In 
Northern Ohio the ayerage temperature during winter is from 27° to 28°. 
In Southern Ohio it is about 385°. 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 rain-fall 
in different 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 
eurve 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- 
