| April. | May. | June. | July. | Aug. | Sept. |
|---|---|---|---|---|---|
| 3.96 | 3.71 | 3.18 | 3.38 | 4.50 | 3.52 |
| Average for the six months, 22.25. | |||||
It will be noticed, that the average for the month of August is about 33 per cent. larger than for June and July. The quantity of rain falling in each month, as registered at the Cambridge Observatory, is as follows:
| Jan. | Feb. | Mar. | Apr. | May. | June. | July. | Aug. | Sept. | Oct. | Nov. | Dec. |
| 2.39 | 3.19 | 3.47 | 3.64 | 3.74 | 3.13 | 2.57 | 5.47 | 4.27 | 3.73 | 4.57 | 4.31 |
| Spring. | Summer. | Autumn. | Winter. | ||||||||
| 10.85 | 11.17 | 12.57 | 9.89 | ||||||||
| Average quantity per year, 44.48. | |||||||||||
The quantity falling from January to July, is much less than falls from July to January.
The great quantity of snow which falls in New England during the Winter months, and is carried off mainly in the Spring, usually floods the low lands, and should be taken into account in establishing the size of pipe to be used in a system of drainage. The following observations of the average depth of snow, have been made at the places cited, and are copied, by Blodget, from various published notices:
| Oxford Co., Me. | 12 | years | 90 | inches | per year. |
| Dover, N. H. | 10 | " | 68.6 | " | " |
| Montreal | 10 | " | 67 | " | " |
| Burlington, Vt. | 10 | " | 85 | " | " |
| Worcester, Mass. | 12 | " | 55 | " | " |
| Amherst, " | 7 | " | 54 | " | " |
| Hartford, Conn. | 24 | " | 43 | " | " |
| Lambertville, N. J. | 8 | " | 25.5 | " | " |
| Cincinnati | 16 | " | 19 | " | " |
| Burlington, Iowa | 4 | " | 15.5 | " | " |
| Beloit, Wisconsin | 3 | " | 25 | " | " |
One-tenth the depth of snow is taken as its equivalent in water, for general purposes, though it gives too small a quantity of water in southern latitudes, and in extreme latitudes too great a quantity. The rule of reduction of snow to water, in cold climates, is one inch of water to twelve of snow.
The proportion of the annual downfall of rain which is collectable into reservoirs—or, in other words, the per-centage of the rain-fall which drains off—is well shown in a table used by Ellwood Morris, Esq., C. E., in an article on "The Proposed Improvement of the Ohio River" (Jour. Frank. Inst., Jan., 1858), in which we find, that, in eighteen series of observations in Great Britain, the ratio, or per cent. of the rain-fall which drains off is 65½, or nearly two-thirds the rain-fall.
Seven series of observations in America are cited as follows:
| No. | Name of Drainage Area. | Annual rain-fall, in inches. | Drainage flowing away, in inches. | Ratio, or per ct. of the rain which drains off. | Authorities. |
|---|---|---|---|---|---|
| 1 | Schuylkill Navigation Reservoirs | 36 | 18 | 50 | Morris and Smith. |
| 2 | Eaton Brook | 34 | 23 | 66 | McAlpine. |
| 3 | Madison Brook | 35 | 18 | 50 | McAlpine. |
| 4 | Patroon's Brook | 46 | 25 | 55 | McAlpine. |
| 5 | Patroon's Brook | 42 | 18 | 42 | McAlpine. |
| 6 | Long Pond | 40 | 18 | 44 | Boston Water Com'rs. |
| 7 | West Fork Reservoir | 36 | 14 | 40 | W. Milnor Roberts. |
| Totals | 269 | 134 | 347 | ||
| Averages | 38 | 19 | 50 |