—32 
height and where it is free from influences such as the wind, 
or the movement of the water, which can affect the true 
level. The height should be read to within 1-300 of the 
depth in order that the error may be within one-half of i per 
cent. 
6. The weir ought to be constructed with care and care¬ 
fully located. It should not vary more than 4 degrees from 
being perpendicular to the channel. Its sill should be hori¬ 
zontal. 
The disturbing causes may be divided into three classes; 
those which always tend to to increase the discharge over the 
computed amount; those which always tend to decrease the 
amount; and those which may either increase or decrease the 
amount, one being as likely to occur as the other, and in the 
long run tending to balance each other. 
The measurement of the depth of water is in general as 
likely to be too great as too small, with careful measurement, 
and the errors due to this may be neglected. 
The effect of obliquity of the weir, or of eddies is to de¬ 
crease the flow below the computed amount. 
The effect of any velocity of the water as it approaches 
the weir, of the nearness of the sides or bottom to the weir, 
incomplete contraction, of a crest not perfectly sharp, of air 
not having accessbeneath the sheet of falling water, etc., the 
effect of each of these is to increase the discharge. 
The causes tending to increase the discharge evidently 
out number those tending to decrease it, and are, all things 
being taken into account, more difficult to overcome. 
It is frequently not possible to meet all the conditions. 
But the errors due to the weir not being vertical, or hori¬ 
zontal, or pe-rpendicular to the current, or for crest not being 
sharp, can be obviated by careful construction. 
If the weir is not vertical, the discharge is increased or 
diminished, according as the inclination may be down or up 
stream. The correction amounts to 4 percent for inclina¬ 
tions as great as one horizontal to three vertical, or for an¬ 
gles of about 18 degrees.^' For less inclinations the correc¬ 
tion would be less. 
The effect of nearness of the sides in increasing the dis¬ 
charge, amounts to about one per cent, when the distance is 
equal to the-depth of the water on the weir, about % of one 
per cent, when the distance is 1)4 times the depth, and may 
be neglected when over twice the depth of water on the weir. 
* From experiments made by M. Bazin, Annales des Fonts at Chaussees, 
Jan. 1890. Translated in Froc. Engrs. Club, of Fhila. by Marichal and Trautwine, 
Also, Trautwines’ Engrs. Focket Book, 16th ed., p. 267 i. 
