it, of sufficient length to secure uniform velocity without internal agitation or eddies.

(2) The opening itself must have a sharp edge on the up-stream face, and the walls cut away so that the thickness shall not exceed one-tenth the depth of the overflow.

(3) The distance of the sill or bottom of the weir from the bottom of the canal shall be at least three times the depth on the weir, and the ends of the sill must be at least twice the depth on the weir from the sides of the canal.

(4) The length of the weir perpendicular to the current shall be three or four times the depth of the water.

(5) The velocity of approach must be small; for small weirs it should be less than 6 in. per second. This requires the channel of approach to be much longer than the weir opening.

(6) The layer of falling water should be perfectly free from the walls below the weir, in order that air may freely circulate underneath.

(7) The depth of the water should be measured with accuracy, at a point back from the weir unaffected by the suction of the flow and by the action of waves or winds.

(8) The sill should be horizontal, the plane of the notch vertical.

FIG. 20a.-Stream gauging in the Hawaiian Islands.

To reiterate, it is very important in making a measurement that the level of the notch or sill of the weir be exactly horizontal and that the head of water be measured with the greatest exactness.

Methods of Measuring the Head.-The head may be measured by readings on a stake, by a hook gauge or by floats. In any case the

measurement should be made far enough back from the weir so that the level will be unaffected by the flow.

A suggestion for setting a stake and reading therefrom is shown in Fig. 20. The leveling board should be perfectly true on top and the legs of exactly equal length, and the stake should be driven until

FIG. 21.Hook-gauge.

the shoulder on it is at the same level as the top of the notch as shown by the spirit level. Or the same result may be obtained with an engineer's level and rod, if available. Or the stake may be driven so that the shoulder is at the water-level at the moment that the rising water just reaches the level of the notch; this method, however, is affected by capillary action at the crest.

The hook gauge (Fig. 21) offers the most accurate method of measuring the head. It consists of a scale 2 ft. long graduated to 100ths ft. and sliding in the groove of a frame which also carries an adjustable vernier reading 1000ths ft. By means of this adjustable vernier the scale can be set to read exactly zero when the tip of the hook is level with the crest of the weir and all readings can be taken directly without the necessity of making a correction for initial reading. The lower end of slide is fitted with a movable brass hook, upper end with a micrometer screw.

It is secured to a stake driven into the stream, and readings are obtained by moving the slide up until the point of the hook just makes a "pimple" on the surface of the

water.

Floats are sometimes used; they are made of hollow metallic vessels, or painted blocks of wood or cork, and carry a vertical stem; on the stem is an index-hand or pointer that moves over a graduated scale.

Computation of Results.-The following table2 gives the discharge in cubic feet per second for each foot in length of weir such as is described above. It gives very approximate results also when there is end contraction (sides boarded), provided that L is at least = 10 H. and but about 6 percent in excess of the truth if L = 4 H. (in contracted weir).

Example. For a weir 5 ft. 0 in. wide, and a head of .74 ft., the discharge will be 2.120 X 5 10.6 cu. ft. per second.

=

To obtain the discharge in U. S. gallons per minute, multiply the above result by 448.831, or 10.6 X 448.83 = 4757.6 gallons per minute.

1 Carpenter's "Experimental Engineering."

2 Taken, by permission, from Trautwine's "Civil Engineer's Pocket-Book."

SEC. IV. INFORMATION TO BE SUPPLIED FOR OBTAINING DESIGNS AND ESTIMATES

STEEL RAILROAD OR HIGHWAY BRIDGE; INFORMATION TO BE SUPPLIED FOR OBTAINING DESIGNS AND ESTIMATES

It is supposed that a railroad company, a municipality or promoters desire designs and estimates for a bridge to cross a certain valley. The following clauses outline the data that should be submitted to the engineers or bridge companies in order that estimates may be figured as closely as possible and be obtained with the least delay and previous. correspondence.

It is presupposed that, on structures of any importance at least, this information will be collected and submitted by an engineer. The comparison of the designs and bids, even for the smaller structures, must, by all means, be passed on by an engineer. The author recently had submitted to him, from a not obscure bridge company, a design for a highway span that made no pretense whatever of complying with the specifications imposed. The bridge was intended for export, and the manufacturer was evidently under the impression that no engineer would pass on the plans submitted to the agents handling the business.

The clauses refer principally to the superstructure, and are intended for aid in obtaining bids on that part only. However, an extended consideration of clause (30) by the contractor or his agent in person, will enable bids to be obtained on the substructure also.

Railroad Bridge

(1) Is track to be standard (4 ft. 8 1/2 in.) or narrow gauge? If the latter, give dimension.

(2) Is bridge to be for a single- or double-track railroad? If single, is provision to be made for future double-tracking?

(3) Under what specifications is the bridge to be designed? (Cooper's, Waddell's, American Maintenance of Way, etc., or Special Specifications.)

(4) If exceptions are to be made to any clauses in the specifications, indicate fully. (5) Give engine and train loading for which bridge is to be designed. Are one or two engines to precede the train?

(6) If railroad is narrow gauge, submit a clearance diagram. If narrow gauge and double track, give distance centre to centre of tracks.

(7) Is any account to be taken in designing the bridge of a future increase in size and weight of rolling-stock? If not covered by any clause in the specifications or in the assumed live load, indicate the requirements definitely.

(8) Submit a blue-print of the profile and plan of the crossing to a convenient scale, showing all the information derived from the consideration of Clauses (9) to (11), etc.

(9) If bridge is to be placed on old piers or abutments, give complete dimensions on drawing.

(10) Is grade at crossing level or rising? Indicate on profile, giving elevation of

base of rail with reference to other elevations, and giving points of rise, and amount of same in percent or feet per mile.

(11) Is alignment at crossing tangent or curved? If the latter, indicate on plan points of curvature, degree of curve, length and alignment of spiral, etc.

(12) Consider general clauses (26) to (43).

Highway Bridge

(13) Clear width of roadway? Are outside sidewalks to be on one side or bridge only or on both? If the former, indicate side required on plan. Clear width of sidewalks?

(14) Is bridge to carry trolley tracks? Single or double track? If single, is any provision to be made for future double-tracking?

(15) Under what specifications is the bridge to be designed?

(16) If exceptions are to be made to any clauses in the specifications, indicate fully. (17) Live Load.—Specify a class of loading as given in Cooper's General Specifications for Steel Highway and Electric Railway Bridges and Viaducts; or, give in detail the road-roller, street car, or other concentrated load to be carried with an additional distributed live load per square foot of unoccupied surface; and an alternate uniformly distributed live load (per lineal foot of bridge). (18) Any special requirements as to headroom?

(19) Is any account to be taken in designing the bridge of a future increase in size and weight of rolling load? If not covered by any clause in the specifications or in the assumed live load, indicate the requirements definitely.

(20) Floor.-Are floor-joists to be of steel or wood? Are stringers for carrying street-car tracks to be of steel or wood? What construction will be used for floor; wood planking, macadam on R. C. floor slabs, macadam on steel buckleplates, etc.? Describe fully so that calculations may be made of dead load. If R. C. floor slabs are to be used, is the manufacturer to furnish the reinforcing metal?

(21) Sidewalks.-Describe construction as outlined above for floors.

(22) Submit a blueprint of the profile and plan of the crossing to a convenient scale, showing all the information derived from the consideration of Clauses (23), (24), etc.

(23) If bridge is to be built on old piers or abutments, give complete dimensions on the drawing.

(24) Is grade at crossing level or rising? For bridges of several spans with rising grade, indicate on profile, line of top-of-rail of street-car tracks with reference to other elevations; if there are no car tracks, indicate surface of roadway at centre.

(25) Consider general clauses (26) to (43).

General Clauses, for both Railroad and Highway Bridges

(26) Will crossing be "square" or "skew"? If the latter, indicate angle on plan. (27) Indicate on profile and give elevations for points of high and low water and flood heights; describe character of stream, whether an estuary, fresh-water river, mountain stream, etc., especially with reference to velocity, floods, scour, character and extent of drift, etc.

(28) Is stream navigable, so that a draw-span may be required? Indicate clear width and headroom to be preserved, either as required by the traffic or as imposed by the War Dept. or other authority. State whether opening of bridge will be frequent, requiring first class equipment; or practically negligible. What power will be used for operating, hand, steam, gas-engine or electric? In case of electric, state whether D.C. or A.C., voltage, phase and frequency (for A.C.).