1 70 The Realm of Nature CHAP. 



other rivers where the fresh water meets the sea gradually 

 in a narrow inlet, the wedge-like action of the salt water at 

 high tide is scarcely perceptible. The effect of the tidal 

 currents sweeping to and fro in the funnel-shaped channel 

 is to mix the river and sea-water together as if they were 

 being shaken in a bottle. In such an inlet as that of the 

 Thames or the Firth of Tay, where the river is large, the 

 water is found to grow rapidly salter from river to sea, the 

 surface is much fresher than the lower layers, and the 

 change of salinity between high and low tide is very marked. 

 This form of river entrance is appropriately called an Estuary. 

 When, however, the inlet is very large compared with the 

 river, and when there is no bar at the opening, the estuar- 

 ine character is only shown at the upper end. In the Firth 

 of Forth, for example, the landward half is an estuary, but 

 in the seaward half the water has become more thoroughly 

 mixed, the salinity is almost uniform from surface to bottom, 

 and increases very gradually toward the sea. The result is 

 that the river-water meets the sea diffused uniformly through 

 a deep mass of water scarcely fresher than the sea itself, so 

 that the two mix uniformly, and the sea becomes slightly 

 freshened throughout its whole depth for many miles from 

 land. 



232. Temperature in River Entrances. The tempera- 

 ture of a river in the temperate zone follows that of the land 

 over which it flows, and is thus subject to considerable 

 variations between day and night. River-water, unless it 

 flows very rapidly, can never become colder than 32; but 

 in summer its temperature may be raised to a very high 

 degree if there is little rain and strong sunshine. Rain 

 lowers the temperature of rivers in summer, especially 

 when it floods torrents descending from cold mountains. 

 Such rivers are warmer than the sea in summer and cooler 

 than the sea in winter. In an estuary or firth in summer 

 the temperature is highest on the surface and in 'the river, 

 diminishing at first very rapidly, but afterwards more slowly 

 as the sea is approached. In autumn, on account of the 

 more rapid chilling of the land, the temperature becomes 

 nearly uniform in river, estuary, and sea, and from surface 



