GRAVITATION

is that system of supply where the rain-water drainage of elevated water-sheds is gathered in natural or artificial storage basins, and conveyed through conduits by gravitation to the point of supply. The important points entering into the consideration of this method are:

1. Character of water; present and future contamination.

2. Water-shed; present and future requirements for quantity and availability, with proper knowledge of the geology of the surrounding country.

3. Rain-fall, absorption and evaporation.

4. Elevation and distance of source.

5. Route of conduit.

6. Cost of construction.

The practical objections to the system are:

1. Contamination of source by surface drainage of cultivated lands; pollution of feeding streams, or growth of vegetation.

2. Necessity for large impounding reservoirs for storage of water during rainy seasons, requiring immense puddled walls, whose stability is questioned.

3. The uncertainty of dependence on the requisite rain-fall, and liability of short supply, or a possibility of water-famine.

4. The large expenditure at the outstart for construction of supply that must be ample for future demands.

Surface waters from calcareous cultivated lands are polluted with but a moderate amount of organic matter; but, as some of this matter is almost always of animal origin, they are always undesirable, and may at any time become dangerous for domestic use.

If necessity compels their use, great care ought to be taken to secure their efficient filtration before they are delivered to consumers. This affords some, though by no means complete, protection from the propagation of zymotic disease through the agency of such waters.

They are generally very hard, and, unless artificially softened, occasion a great waste of soap when used for washing. Of all the waters of this description, those which flow from the surface, or from the drains of sewage farms, are generally most impure, because the time during which the foul sewage is exposed to the purifying action of plant and soil is reduced to a minimum.

Surface water from non-calcareous soil is generally soft but usually turbid and subject to animal contamination. Such water should always be carefully filtered.

ANALYSIS OF LAND DRAINAGE WATER FROM SEWAGE FARMS (PARTS BY WEIGHT OF 100,000 PARTS).



	TOTAL IMPURITIES.	ORGANIC CARBON.	ORGANIC NITROGEN.	CHLORINE.	PREVIOUS SEWAGE OR ANIMAL CONTAMIN- ATION.	HARDNESS.

Worst Condition.	94.	2.160	.274	13.10	10.090	35.58
Best “	24.60	.108	.055	4.05	17.920	9.20
Average “	64.02	.982	.191	6.36	10.443	33.09

Much depends upon the knowledge of the climatic influences and rain-fall, extended, as it should be, through years of observation in determining the available quantity of water. Engineers, however, are liable to be too sanguine of the resources from water-sheds, by assuming, as a general rule, the average, rather than the minimum, rain-fall.

In 1868 nearly all the cities and towns of England, supplied by gravitation, suffered a water-famine, because of the overestimate of the available rain-fall, and in an insufficient provision of storage for an unusually long drought. Although the rain-fall for the year was above the average, yet it was unequally distributed.

The authorities of Manchester were obliged to publish official notices cautioning the inhabitants against waste, and, on the 3d of August, limited the supply to the city to twelve hours of the day, stopped the street watering, and diminished the trade supplies by one-half. In the middle of September the general supply of the town was further limited to eight hours per day. Many persons were prosecuted for waste or undue use of water.

Liverpool, Sheffield, Bristol, and several other large cities were obliged to resort to like severe methods enforced at Manchester. New York has been using every gallon that the aqueduct is capable of supplying; and, during the drought of last summer, when the head of water at Croton Lake was diminished, the capacity of the aqueduct was so reduced that the flow of water to the city was reduced, and a water-famine averted only by a Providential rain-fall.

The rule observed among engineers, in Great Britain, in determining the calculated rain-fall, is the deduction of one-sixth from the average rain-fall of twenty years for an average annual rain-fall of the three driest consecutive years in that period. But, as Mr. Homersham, C. E., observes, the axiom in mechanics, that the strength of a beam is the strength only of its weakest parts, applies also to gravitation water-works, their real strength or power of supply being only the minimum quantity they may be reduced to.

Allowance for absorption depends upon the geological formation and stratification, and for evaporation, upon local influences.

The following is taken from Hughes’ Water-Works:

“A flat, low-lying country is seldom well adapted for the impounding of water by embanking across the valleys. In such a district, long and shallow embankments would be required, and these would cause the water to spread out over a great area with a very shallow depth. Under these circumstances, the water is apt to vegetate and become highly impure. Again, in low-lying districts of flat countries the rain-fall is seldom nearly so great as in upland districts, so that much larger drainage areas must be sought.”

In addition to the general configuration of the valleys, which ought to be deep and with precipitous sides, flanked by lofty hills, there are several other points which require attentive examination in projects for collecting water from drainage areas:

1. The area of water-shed.

2. The geological character of the soil as affecting its capacity to absorb rain, and to allow the infiltration of water through it.

3. The character of the surface soil as affording soluble ingredients which may be taken up by the water and serve to contaminate its quality. In this point of view, districts of decomposing peat, districts of arable agricultural land richly manured, and places thickly covered with population, are often highly objectionable.

4. The rain-fall of the district, and especially the minimum fall in any one year.

5. The nature of the surface-soil as affording facilities for procuring puddle and constructing retentive reservoirs.

6. The consideration of compensation to mill-owners and possibly to land-owners where the water is used for irrigation.

The geological structure is extremely important in estimating the capacity of a drainage area. It is not alone the rain which falls on the sloping surface of the hills, and finds its way by gravitation to the lower levels; but the effect of springs is also very great in augmenting the quantity of water. Many drainage areas are also valleys of elevation, in which the strata dip in opposite or anticlinal directions on opposite sides of the valley. In this case it is evident that much of the rain falling on a porous surface will insinuate itself between the partings of the strata, and flow off in a direction contrary to that of the surface drainage.

From Mr. Beardmore’s work we take the following, as the proportion or percentage of rain-fall which flows off the surface:

“From twenty examples we have 89 as the largest per centage, the lowest 29 per cent., and the average 64 per cent.

“The Eaton Brook water-shed, in Madison County, New York, of 6,800 acres, with steep slope and compact soil, underlaid by hard greywacke rock, elevated 1,350 feet above the sea, availed 66 per cent. of the rain-fall as surface flow.

“A similar water-shed, Madison Brook, gave 50 per cent. Experiments by Wm. McAlpine, for Albany water-works, shows that from a water-shed of 2,600 acres, 41½ per cent. of the rain-fall was carried off by the streams from May till October, inclusive, while from November till April, 77.6 per cent. was so carried off.”

In England the allowance for absorption and evaporation ranges from nine to nineteen inches per annum. In this country it is from 75 to 100 per cent. greater.

We produce from “Fanning’s Water Supply” the following table of experiments on evaporation from surfaces of shallow tanks:

Cambridge—Length of trial,		one year;	evaporation in inches,		56.00
Salem	“	“	“	“	56.00
Syracuse	“	“	“	“	50.20
Ogdensburgh	“	“	“	“	49.37
Dorset, England	“	three years	“	“	25.92
Oxford “	“	five “	“	“	31.04
Bombay	“	five “	“	“	82.28
Croton	average,	six, “	mean evap. equal 81 }		39.21
Croton	average,	six, “	per cent. of rain-fall. }		39.21
Lea Bridge, London	“	seven “	average rain-fall		27.7
			annual evap. min.		12.067
			“ “ max.		25.141

The following from the same author of the minimum flow of streams in cubic feet per second, per each square mile of water-shed:

From	1 square mile	.083	From	250 square miles	.25
From	10 square miles	.1	From	500 square miles	.40
From	25 square miles	.11	From	1,000 square miles	.35
From	50 square miles	.14	From	1,500 square miles	.38
From	100 square miles	.18	From	2,000 square miles	.41

From the different surfaces, its ratio of the annual rain, including floods and flow of springs, is approximately as follows:

	PER CENT.
From mountain slopes or steep rocky hills,	80 to 90
From wooded swamp lands,	60 to 80
From undulating pasture and woodland,	50 to 70
From flat cultivated land and prairie,	45 to 60

MONTHLY EVAPORATION FROM RESERVOIR.

(From Fanning.)


	JAN.	FEB.	MAR.	APR.	MAY.	JUNE.

Mean ratio—inches	.30	.35	.50	.80	1.45	1.70


	JULY.	AUG.	SEPT.	OCT.	NOV.	DEC.

Mean ratio—inches	1.85	2.00	1.45	.75	.50	.35

AVERAGE AVAILABLE RAIN-FALL FOR STORAGE PURPOSES.

(From Fanning.)


	JAN.	FEB.	MAR.	APRIL.	MAY.	JUNE.

Gain by rain—inches	2.00	2.21	2.40	2.93	3.47	2.88
Loss by evaporation—in.	.60	.70	1.00	1.60	2.90	3.40
Difference—Gain inches	1.40	1.51	1.40	1.33	.57	—
Difference—Loss inches	—	—	—	—	—	.52


	JULY.	AUG.	SEPT.	OCT.	NOV.	DEC.	TOTAL.

Gain by rain—inches	2.99	3.00	2.67	2.53	2.48	2.24	32.
Loss by evaporation—in.	3.70	4.00	2.90	1.50	1.00	.70	24.
Difference—Gain inches	—	—	—	1.03	1.48	1.54	8.
Difference—Loss inches	.71	.80	.23	—	—	—	—

SUMMARY OF FLOW OF RAIN-FALL IN CU. FT. PER MINUTE PER SQUARE MILE.—(From Fanning.)

	AT LAKE	AT CROTON		AT LAKE	AT CROTON
	COCHITUATE	BASIN		COCHITUATE	BASIN
	CU. FEET.	CU FEET.		CU. FEET.	CU FEET.
January,	99.17	92.48	July,	45.27	48.37
February,	150.42	147.69	August,	49.15	70.22
March,	174.76	177.02	September,	42.84	85.99
April,	169.80	132.63	October,	62.45	81.08
May,	131.80	164.49	November,	75.90	124.92
June,	44.27	115.12	December,	78.94	106.23