Ware River, Mass. | 1.62| 33| 71 | ... | 785| 11,190 Sodom, N. Y. | ....| ..| 72 | 89 | 500| 9,500 Reservoir No. 5, Boston | | | | | | water-works | 1.91| 19| 65 | 70 |1,865| 7,438 Titicus, N. Y. | ....| ..| 105 | 115 |.....| 7,000 Hobbs Brook, Cambridge | | | | | | water-works | 1.00| 12| 23 | ... |.....| 2,500 Cochituate, Boston water-works| 1.35| 8| .. | ... |.....| 2,160 Reservoir No. 6, Boston | | | | | | water-works | 0.29| 25| 52 | ... |1,500| 1,500 ------------------------------+-----+---+-------+------+-----+------- =179. Overturn of Contents of Reservoirs Due to Seasonal Changes of Temperature.=—It will be noticed that the average depth is less than about 20 feet in few cases only. If the water is deep, its mean temperature throughout the year will be lower than if shallow. During the warmer portion of the year the upper layers of the water are obviously of a higher temperature than the lower portions, since the latter receive much less immediate effect from the sun’s rays. As the upper portions of the water are of higher temperature, they are also lighter and hence remain at or near the top. For the same reason the water at the bottom of the reservoir remains there throughout the warm season and until the cool weather of the autumn begins. The top layers of water then continue to fall in temperature until it is lower than that of the water at the bottom, when the surface-water becomes the heaviest and sinks. It displaces subsurface water lighter than itself, the latter coming to the surface to be cooled in turn. This operation produces a complete overturning of the entire reservoir volume as the late autumn or early winter approaches. It thus brings to the surface-water which has been lying at the bottom of the reservoir all summer in contact with what vegetable matter may have been there. The depleted oxygen of the bottom water is thus replenished with a corresponding betterment of condition. It is the great sanitary effort of nature to improve the quality of stored water entrusted to its care, and it continues until the surface is cooled to a temperature perhaps lower than that of the greatest density of water. Another great turn-over in the water of a lake or reservoir covered with ice during the winter occurs in the spring. When the ice melts, the resulting water rises a little in temperature until it reaches possibly its greatest density at 39°.2 Fahr., and then sinks, displacing subsurface water. This goes on until all the ice is melted and until all water cooled by it, near the surface, below 39°.2 Fahr. has been raised to that temperature. The period of summer stagnation then follows.