If the district hydrographer desires he can compute discharge from the formula given on page eighty-six of "Instructions relating to the work of the United States Geological Survey, May 1, 1903," namely, 6b 8

(with altered symbols); Q' =(a+b+c) LV,. The letters have the

same meaning as in the formula on page 47, except that Q' is the discharge through the vertical section extending from halfway between a and b to halfway between b and c.

It sometimes happens that the velocity becomes very small or "0" in some parts of the cross-section at a station, as in the case of the Salina station for the part from 95 to 102 feet from the initial point. This area of small or "0" velocity is sometimes neglected in computing the cross-sectional area and the mean velocity. Mean velocity obtained in this way is sometimes misleading, because it may make a section in which the true velocity is only 0.3 or 0.4 foot per second (a section that should not be used) appear to have a velocity of half a foot or more per second. There is danger, too, that if this low velocity area is neglected in computing mean velocity, no attention will be given to measuring those velocities, and thus an error will be introduced into the discharge. It has been decided to include the whole area in the column headed "area" in computing mean velocity.

COMPUTATIONS OF VERTICAL-VELOCITY CURVES AND COEFFICIENTS.

The method of making vertical-velocity-curve observations is described on page 20, the form for recording the observations is given on page 50, the velocities in column 4 are plotted as shown on page 51, the depth of center of meter below the surface being used as ordinates and velocities as abscissas. A smooth curve is drawn among them making a graphic adjustment of the observations. The mean abscissa of this curve is the mean velocity in this vertical. The depth below the surface of the thread of mean velocity is the ordinate which corresponds to the mean abscissa or to the computed mean velocity.

To compute the mean velocity from the vertical-velocity curve, divide the depth into from 5 to 10 equal parts and write the velocity at the center of each part in column 6, headed "velocity from curve." Find the sum of these and divide by the number of parts; the quotient is the mean velocity in that vertical.

It is often more convenient, when the depth is a number of feet and a fraction, as 8.3 feet, to divide the depth into 8 parts of a foot width, and a part of 0.3 foot width. Then the velocity to enter in column 6 for the narrow part is 0.3 of the velocity at the center of it. The velocities at a point 1 foot below the surface, mid-depth, and bottom are read directly from the vertical-velocity curve, and recorded in column 8. The "coefficients for reducing to mean velocity," required in column 9, are obtained by dividing the mean velocity by each of the velocities in column 8.

IRR 94-04- 4

Vertical-velocity measurement made November 2, 1903, by E. C. Murphy, meter No. 585 feet from initial point

AND HOLLISTER.

338, on Susquehanna River, at Harrisburg, State of Pennsylvania. Measurements at for soundings. Depth 8 ft.

mean 3.08 ft. Channel open. Thickness of ice, ft.

As the months are of varying length it is necessary to use three or four factors to convert the average discharge for the month in secondfeet into the total in acre-feet. One second-foot flowing for twentyfour hours is equivalent to 86,400 cubic feet. Since there are 43,560 square feet in an acre there will be the same number of cubic feet in an acre-foot. Dividing, it is found that 1 second-foot for twenty-four hours very nearly equals 2 acre-feet, or, in exact figures, 1.983471 acrefeet. This multiplied by the number of days in the month will give the total monthly discharge in acre-feet. This quantity, therefore, must be multiplied by 28 for the month of February. or 29 for that month in leap year, and by 30 or 31 for the other months.

For the month of February when it has 28 days the factor to be used is 55.537188. For convenience in computation this factor multiplied from 1 to 9 is given in the following table:

When February has 29 days the factor to be used is 57.520659. This when multiplied from 1 to 9 gives the following:

For the months containing 30 days, viz, April, June, September, and November, the factor to be used is 59.504130. This, when multiplied by the unit figures, gives the following results:

For the months containing 31 days, viz, January, March, May, July, August, October, and December, the factor to be used is 61.487601. This, when multiplied by the unit figures, gives the following results: