The suspended car (illustrated in Fig. 123) consists simply of a stout box about 3 feet wide and 5 feet long hung from pulleys running on the wire cable. The stream gauger, sitting in the car, moves himself from side to side of the river and uses his meter as from a boat, lowering or raising it by hand or by a rope and pulley. This has been found very satisfactory. All parts of this apparatus are made of great strength, the cable being capable of holding from ten to fifty times the greatest load put upon it.

Fig. 123.-Method of using the current meter from suspended car.

In the cable and traveler method the main feature is that the observer stays on shore, and has no necessity, after the cables are once in position, of going out upon or over the water. The apparatus as used by the Survey was devised by Mr. Hall, and put into practical operation on the Carson and Tuckee Rivers in Nevada by Mr. Trowbridge, hydrographer for that division, who commends it highly, and states that

in his opinion it is the best system, especially for torrential mountain streams.

In general principle this apparatus is similar to that used in the gauging of the river Severn in 1880, the most notable difference being in the manner in which the meter is held from being carried downstream. In the first device the cord which steadied the meter was held in place by a cast-iron anchor plate weighing 70 pounds which rested in the bed of the river, while in the Hall apparatus the meter is held from being swept backwards by a stay line running upstream.

The meter is run out on the main cable by means of halyards, and is lowered and raised by a double incandescentlight cord, or insulated wire, which serves the two purposes of conveying the electric current from the battery and instrument on shore to the meter and of acting as a suspending and sounding cord. The stay line just mentioned is fastened to a pulley which travels on a second smaller cable crossing the river above the main cable. The lower end of this stay holds the meter from being swept downstream. By manipulating the various ropes the meter can be placed and held in any position in the cross-section, the revolutions being reported by an electric sounder near the observer.

Soundings can be made in the same manner, quickly and accurately, the rod being held in a vertical position by the stay line, and the depth, if necessary, read off by the amount of movement of the cords on shore, the sounding and insulated circuit cord being graduated to correspond with the marks on the sounding rod.

With careful training and experience one man is able to do all the work of gauging more easily, it is probable, by this method than by any other. As stations are at present located, however, in most of the basins, the necessities of transportation and subsistence require that the hydrographer or steam gauger shall have an assistant; and even at stations which can easily be reached by rail it is found to be economic to employ the resident gauge observer for the few hours during which the.

1 Minutes of Proceedings of the Institution of Civil Engineers, 1885, vol. 80, p. 327. 11 GEOL., PT. 2

-2

gauging is made, as helper or recorder of notes, as his regular compensation at best is very small.

In addition to the cables, wires, and other apparatus necessary for measuring the discharge, there is required some means of measuring the height of the water, that is, a gauge rod of some kind. Usually this is a stick of timber, or heavy scantling, fastened securely to the bank and at the general inclination of the shore. This when in position is marked by means of a level into divisions corresponding to vertical feet and tenths. In favorable situations, as on vertical banks or a bridge pier, an upright rod marked to feet and tenths is placed, but these localities are few. In general it is necessary to establish an inclined gauge, as a vertical rod or pile would not only be in danger of being washed away in high water, but the shore line having retreated at that time the observer could not get sufficiently near the gauge to read it. With the inclined gauge the reading is always at the shore line, and driftwood is not liable to catch upon it.

As soon as this gauge or "nilometer" rod is set, the observer is instructed in the routine of noting down, once, twice, or three times a day, the height of water on the gauge, and the observations should then be continued throughout the year.

DIURNAL VARIATION.

While the hydrographer is putting up the cable and other apparatus for river gaugings, or at the first opportunity, a series of observations should be made to determine the diurnal range, if any, of the river height. In most of the rivers of the arid region, especially near their headwaters, this daily range is quite noticeable, and it is of considerable importance to determine the time of day at which the maximum and the minimum usually occur. This alternation of high and low water is caused by changes of temperature, the heat of the day melting the snow and the cold during the night checking this, or even freezing a portion of the water. The time at which this effect is shown at any given point on the stream below is evidently dependent upon the distance from the source of sup

ply. The maximum flow at one place may occur at midnight, lower, down stream it will be early in the morning, and still farther away the highest point will be at noon.

The main stream formed by the union of many such tributaries does not show such a decided daily range, as the maximum and minimum points of the various streams do not coincide and tend to neutralize one another.

The number of gauge readings per day is determined largely by this preliminary examination. If the stream does not fluctuate noticeably during the day, a single observation for that period is sufficiently accurate for all purposes, provided the observer takes careful note of any unusual or sudden flood. On the other hand, it is obvious that a single observation will be deceptive if this happens to fall each day at the usual time. of high or of low water; at least two observations are then necessary to determine the mean daily height.

For greater accuracy, a self-registering gauge1 or nilometer is used. This consists of a cylinder or dial driven by clockwork at such a rate that one complete revolution is made in a week. A marking device connected with a float, which rises and falls with the river, traces on paper placed on this cylinder a line which, by its position, gives the height of water at any hour of day or night. A record of this character gives data, not only for determining the daily mean flow with great precision, but also the maximum and minimum discharges.

RATING THE STATION.

After the station is equipped and daily gauge readings begun, the rating of the station is undertaken. This is the main and most expensive operation of the hydrographic work, as it is necessary to measure the discharge at various heights. of water, both at high and low stages, as well as at intermediate points. The measurements are made by the hydrog 'rapher or his assistants, who visit each station from time to

'One form described in Fourth Biennial Report of State engineer of Colorado, 1888, p. 54. Another form is illustrated in the Second Annual Report of the Territorial engineer of Wyoming, 1890.

time in their round of the basin. To get a complete series of measurements is thus usually a matter of several months, as the height of the river can not be predicted except in the most general way. The hydrographer and his party can not remain encamped on the banks of a stream waiting for the variations of high and low water, but must take his chances of finding the river at the stage at which he desires to measure it when he arrives.

The results of these measurements, finally obtained, are plotted on rectangularly ruled paper, the gauge height of the river as ordinates, the corresponding discharge as abscissæ. These points generally lie in such a position as to suggest the path of a simple curve. This line is therefore usually sketched by eye, the hydrographer from his knowledge of circumstances giving intuitively more weight to certain observations than to others. On Pl. LXX is given an illustration of one of these rating curves. The points obtained by observation are indicated by dots surrounded by circles and numbers in order of dates of measurement. The finer rulings of the paper ordinarily used have been omitted in order to make the position of the dots more apparent to the eye. When the curved line has finally been adjusted, the rating table is constructed. This is a table giving the discharge in cubic feet per second for each tenth of foot height on the gauge rod of a station. Knowing then the height of the river at any time, the corresponding discharge can be at once read off from this table. This rating table is taken directly from the plotted curve, the discharge corresponding to each tenth of foot in height being obtained by the position of the curve. It is advisable, as affording a convenient check on the numerical accuracy, to note down in a column at the right the differences between successive discharges, i. e., the increase of discharge each time due to rise of 01 foot. If these at any place become constant the discharge curve evidently has flattened into a tangent, and if the differences decrease, the curve is reversed and should be slightly changed or adjusted, so that the differences show a constant, though slight, increase. There may one exception to this rule. It is conceivable that a river may,

be