Farm drainage Part 6

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 quant.i.ty of water in southern lat.i.tudes, and in extreme lat.i.tudes too great a quant.i.ty. 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 65-1/2, or nearly two-thirds the rain-fall.

Seven series of observations in America are cited as follows:

========================================================================== Name Annual Drainage Ratio, or of rain-fall, flowing per ct. of No. Drainage Area. in inches. away, in the rain Authorities.

inches. which drains off. ---+---------------+----------+--------+-----------+---------------------- 1 Schuylkill Navigation Reservoirs 36 18 50 Morris and Smith.

2 Eaton Brook 34 23 66 } 3 Madison Brook 35 18 50 }McAlpine.

4 Patroon's Brook 46 25 55 } 5 " " 42 18 42 } 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 ==========================================================================

These examples show an average rain-fall of thirty-eight vertical inches, and an annual amount, collectable in reservoirs, of nineteen inches, or fifty per cent.

The per-centage of water of drainage from land under-drained with tile, would be greater than that which is collectable in reservoirs from ordinary gathering-grounds.

If a soil were perfectly saturated with water, that is, held as much water in suspension as possible to hold without draining off, and drains were laid at a proper depth from the surface, and in sufficient number to take off all surplus water, then the entire rain-fall upon the surface would be water of drainage--presuming, of course, the land to be level, and the air at saturation, so as to prevent evaporation. The water coming upon the surface, would force out an equal quant.i.ty of water at the bottom, through the drains--the time occupied by the process, varying according to the porous or retentive nature of the soil; but in ordinary circ.u.mstances, it would be, perhaps, about forty-eight hours. Drains usually run much longer than this after a heavy rain, and, in fact, many run constantly through the year, but they are supplied from lands at a higher level, either near by or at a distance.

If, on the other hand, the soil were perfectly dry, holding no water in suspension, then there would be no water of drainage until the soil had become saturated.

Evaporation is constantly carrying off great quant.i.ties of water during the warm months, so that under-drained soil is seldom in the condition of saturation, and, on account of the supply by capillary attraction and by dew, is never thoroughly dry; but the same soil will, at different times, be at various points between saturation and dryness, and the water of drainage will be consequently a greater or less per centage of the rain-fall.

An experiment made by the writer, to ascertain what quant.i.ty of water a dry soil would hold in suspension, resulted as follows: A soil was selected of about average porosity, so that the result might be, as nearly as possible, a mean for the various kinds of soil, and dried by several days' baking. The quant.i.ty of soil then being carefully measured, a measured quant.i.ty of water was supplied slowly, until it began to escape at the bottom. The quant.i.ty draining away was measured and deducted from the total quant.i.ty supplied. It was thus ascertained that one cubic foot of earth held 0.4826+ cubic feet of water, which is a little more than three and one-half gallons. A dry soil, four feet deep, would hold a body of water equal to a rain-fall of 23.17 inches, vertical depth, which is more than would fall in six months.

The quant.i.ty which is not drained away is used for vegetation or evaporated; and the fact, that the water of drainage is so much greater in proportion to the rain-fall in England than in this country, is owing to the humidity of that climate, in which the evaporation is only about half what it is in this country.

The evaporation from a reservoir surface at Baltimore, during the Summer months, was a.s.sumed by Colonel Abert to be to the quant.i.ty of rain as two to one.

Dr. Holyoke a.s.signs the annual quant.i.ty evaporated at Salem, Ma.s.s., at fifty-six inches; and Colonel Abert quotes several authorities at Cambridge, Ma.s.s., stating the quant.i.ty at fifty-six inches. These facts are given by Mr. Blodget, and also the table below.

QUANt.i.tY OF WATER EVAPORATED, IN INCHES, VERTICAL DEPTH.

Whitehaven, England, Ogdensburg, Syracuse, mean of N. Y., N. Y., 6 years 1 yr. 1 year --------+-------------+-------------+------------ _Jan._ 0.88 1.65 0.67 _Feb._ 1.04 0.82 1.48 _Mar._ 1.77 2.07 2.24 _Apr._ 2.54 1.63 3.42 _May._ 4.15 7.10 7.31 _June._ 4.54 6.74 7.60 _July._ 4.20 7.79 9.08 _Aug._ 3.40 5.41 6.85 _Sept._ 3.12 7.40 5.33 _Oct._ 1.93 3.95 3.02 _Nov._ 1.32 3.66 1.33 _Dec._ 1.09 1.15 1.86 --------+-------------+-------------+------------ _Year._ 30.03 49.37 50.20

The quant.i.ty for Whitehaven, England, is reported by J. F. Miller. It was very carefully observed, from 1843 to 1848--the evaporation being from a copper vessel, protected from rain. The district is one of the wettest of England--the mean quant.i.ty of rain, for the same time, having been 45.25 inches.

This shows a great difference in the capacity of the air to absorb moisture in England and the United States; and as evaporation is a cooling process, there is greater necessity for under-draining in this country than in England, supposing circ.u.mstances in other respects to be similar.

Evaporation takes place at any point of temperature from 32, or lower, to 212--at which water boils. It is increased by heat, but is not caused solely by it--for a north-west wind in New-England evaporates water, and dries the earth more rapidly than the heat alone of a Summer's day; and when, under ordinary circ.u.mstances, evaporation from a water-surface is slow, it becomes quite active when brought in close proximity to sulphuric acid, or other vapor-absorbing bodies.

The cold which follows evaporation is caused by a loss of the heat which is required for evaporation, and which pa.s.ses off with the vapor, as a solution, in the atmosphere; and as heat leaves the body to aid evaporation, it is evident that that body cannot be cooled by the process, below the dew-point at which evaporation ceases. The popular notion that a body may be cooled almost to the freezing-point, in a hot Summer day, by the action of heat alone, is, then, erroneous. But still, the amount of heat which is used up in evaporating stagnant water from undrained land, that might otherwise go towards warming the land and the roots of crops, is a very serious loss.

The difference in the temperature of a body, resulting from evaporation, may reach 25 in the desert interior of the American continent; but, in the Eastern States, it is not often more than 15.

The temperature of evaporation is the reading of a wet-bulb-thermometer (the bulb being covered with moistened gauze) exposed to the natural evaporation; and the difference between that reading and the reading of a dry-thermometer, is the expression of the cold resulting from evaporation.

When the air is nearly saturated, the temperature of the air rarely goes above 74; but, if so, the moisture in the air prevents the pa.s.sing away of insensible perspiration, and the joint action of heat and humidity exhausts the vital powers, causing sun-stroke, as it is called. At New York city, August 12th to 14th, 1853, the wet-thermometer stood at 80 to 84; the air, at 90 to 94. The mortality, from this joint effect, was very great--over two hundred persons losing their lives in the two days, in that city.

From very careful observations, made by Lorin Blodget, in 1853, at Was.h.i.+ngton, it was found that the difference between the wet and dry thermometer was 18-1/2 at 4 P. M., June 30th, and 16 at 2 P. M. on July 1st--the temperature of the air being 98 on the first day, and 95 on the second; but such excesses are unusual.

The following table has been compiled from Mr. Blodget's notice of the peculiarities of the Summer of 1853:

The dates are such as were selected to ill.u.s.trate the extreme temperatures of the month, and the degrees represent the differences between the wet and dry thermometer. The observations were made at 3 P. M.:

_Locality._ _Dates._ _Differences._

JUNE, 1853.

Burlington, Vt. 14th to 30th ranged from 8 to 17 Montreal 14th to 30th " 6 to 17 Poultney, Iowa 10th to 30th " 9 to 16 Was.h.i.+ngton 20th to 30th " 8.5 to 16 Baltimore 13th to 30th " 7.4 to 20.2 Savannah 13th to 30th " 5.2 to 17.3 Austin, Texas 10th to 30th " 4 to 24 Clarkesville, Tenn. 4th to 30th " 10.3 to 20.5

AUGUST.

Bloomfield, N. J. 9th to 14th " 5 to 15 Austin, Texas 6th to 12th " 0 to 19 Philadelphia 10th to 15th " 8 to 14 Jacksonville, Fla. 10th to 15th " 6 to 8

Observations by Lieut. Gillis, at Was.h.i.+ngton, give mean differences between wet and dry thermometers, from March, 1841, to June, 1842, as follows:

Observations at 3 P. M.:

_Jan._ _Feb._ _Mar._ _Apr._ _May._ _June._ 3.08 4.40 6.47 5.37 7.05 8.03

_July._ _Aug._ _Sept._ _Oct._ _Nov._ _Dec._ 8.89 5.29 5.63 4.61 4.77 2.03

A mean of observations for twenty-five years at the Radcliffe Observatory, Oxford, England, gives a difference between the wet and dry thermometer equal to about two-thirds the difference, as observed by Lieutenant Gillis, at Was.h.i.+ngton.

On the 12th day of August, 1853, in Austin, Texas, the air was perfectly saturated at a temperature of 76, which was the dew-point, or point of the thermometer at which dew began to form. The dew-point varies according to the temperature and the humidity of the atmosphere; it is usually a few degrees lower than the temperature of evaporation--never higher.

From observations made at Girard College, by Prof. A. D. Bache, in the years 1840 to 1845, we find, that for April, 1844, the dew-point ranged from 4 to 16 lower than the temperature of the air; in May, from 4 to 14 lower; in June, from 6 to 20 lower; in July, from 4 to 17; in August, from 6 to 15 lower; and in September, from 6 to 21 lower.

The dew-point is, then, during the important months of vegetation, within about 20 of the temperature of the air. The temperature of the dew-point, as observed by Prof. Bache, was highest in August, 1843, being 66, and lowest in January, 1844, being 18; in July, 1844, it was 64, and in February, 1845, it was 25. Its hourly changes during each day are quite marked, and follow, with some degree of regularity, the changes in the temperature of the air; their greatest departure from each other being at the hottest hour of the day, which is two or three hours after noon, and the least at the coldest hour which is four or five hours after midnight. The average temperature of the dew-point in April, May, and June, 1844, was, at midnight, 50-1/2, air, 57; five hours after midnight, dew-point, 49, air 54; three hours after noon, dew-point, 54, air, 63-1/2. The average temperature for July, August and September, was, at midnight, dew-point, 58-1/2, air, 65; five hours after midnight, dew-point, 58, air, 62; three hours after noon, dew-point, 60-1/2, air, 78. The average temperature for the year was, at midnight, dew-point, 42, air, 48; five hours after midnight, dew-point, 41, air, 46; three hours after noon, dew-point, 44-1/2, air, 59.

The relative humidity of the atmosphere, or the amount of vapor held in suspension in the air, in proportion to the amount which it might hold, was, in the year 1858, as given in the journal of the Franklin Inst.i.tute, for

_Philadelphia._ _Somerset Co._ April 49 per cent. -- 2 P. M.

May 59 " 72 "

June 55 " 63 "

July 50 " 61 "

August 55 " 58 "

September 50 " 57 "

The saturation often falls to 30 per cent., but with great variability.

Evaporation goes on most rapidly when the per centage of saturation is lowest; and, as before observed, the cause of the excess of evaporation in this country over that of England is the excessive humidity of that climate and the dryness of this. It has also been said that there is greater need for drainage in the United States on this account; and, as the warmth induced by draining is somewhat, in its effect, a merchantable product, it may be well to consider it for a moment in that light.

First: The drained land comes into condition for working, a week or ten days earlier in the Spring than other lands.