362 



HYDRAULICS AND ITS APPLICATIONS 



pegs (J iron nails) at a Imown distance d inches apart, with a scale for 



measuring the distance from the base line of the point of intersection of 



the ripples formed. Calling this distance I (Fig. 161), the following 



equations were found to give the surface velocity in feet per second. 



For d = 6", v = '40 + '206 /. 



d = 4", v = '40 + -280 I. 



With d = 6" and with a velocity of *8 feeb per second the value of I is 

 about 2 inches, while with a velocity of 3*5 feet per second, I is 15 

 inches. 



This method would appear to he capable of results at least as accurate 

 as those obtained by the use of surface floats, and possibly more so because 

 of the greater possibilities of accuracy in the determination of the area of 



the stream at one definite cross 

 section. 



The Pitot Tube. The velo- 

 city at any point in the cross 

 section may also be estimated 

 by means of the Pitot tube 

 (Art. 68), p. 217. 



This method is not so well 

 adapted for measuring such 

 low velocities as usually occur 

 in open channels as for higher 

 velocities such as are more 

 common in pipe flow. 



For small, shallow, and rapid 

 streams it is, however, capable 



of giving fairly good results. Fig. 162 shows a tube, with positive and nega- 

 tive pressure openings as used for the rating of such streams by the United 

 States Geological Survey. This tube was rated in still water in a reservoir 

 and in moving water, being placed for the latter rating at about 30 points 

 in the cross section of a channel 1 foot wide and '6 feet deep. The former 

 rating gave values of C in the formula V = C V '2 g h ranging from '855 

 to "87, with a mean value of *86. The velocities as found by this tube from 

 the still water rating, were invariably greater than were given by the moving 

 water rating the average difference being 6'4 per cent. 1 This great difference 

 is, however, probably due partly to the comparatively large disturbing 

 effect of the tube in such a small channel, and partly to the impossibility 

 of taking measurements near the walls where the velocity is least. 



1 Enyineenng News, vol. 62, No. 7, p. 174. 



FIG. 161. 



