S/K WILLIAAf. SIEAfENS t F.K.S. 283 



and of the counter would retard it most at low speeds, and the 

 friction of the vaues iu gliding through the water (which increases 

 in the ratio of the square of the velocity) would again greatly 

 retard it at high speeds, the maximum rate of measurement being 

 obtained at a medium speed. 



By the addition of the second, or left-handed drum, these 

 variations in speed are, however, very perfectly compensated. For 

 the sake of illustration, let it be imagined that both screw drums 

 revolve independently of each other (of course in opposite direc- 

 tions), and that the second or left-handed one alone imparts iu 

 motion to the dial ; let it also be supposed that the friction of 

 both drums is the same ; the water, in meeting the oblique vaned 

 of the first drum in a direction parallel to the axis, will be deflected 

 from its straight course proportionally to the resistance to rotation 

 of the drum, say an angle of 1, as shown at A in Fig. 5, Plate 20. 

 Pursuing its fresh course, it will strike the left-handed screw 

 blades of the registering drum at B, in an angle at 1 more obtuse 

 than the previous, and being deflected by the resistance offered 

 through 1 in the opposite direction, it follows that the water 

 passes out in a direction parallel to the axis, as at C, and, conse- 

 quently, that a true rate of measurement is obtained. The con- 

 dition of both drums being perfectly alike, it follows that the same 

 compensation must be effected at all speeds. Nor is this com- 

 pensating effect disturbed by coupling both drums rigidly together 

 by bevil gearing, whereby a great practical advantage is obtained, 

 that, namely, of one drum assisting powerfully to overcome an 

 obstruction offered to the other. Let it be imagined, for instance, 

 that a pebble or piece of vegetable matter has wedged itself between 

 the casing and tip of the vane of the first drum, so as to stop it 

 entirely, and to force the column of water passing through into the 

 helical course ; the water would then impinge upon the left-handed 

 vanes of the second drum rectangularly (supposing the inclination 

 of the reverse vanes to be at an angle of 45 to the axis), and ex- 

 pend its entire momentum upon it, the effect of which would be 

 added to the impact on the first drum through the bevil gearing, 

 to overcome the obstruction. The motion is conveyed to the 

 counter by the upper bevil wheel N, but the opposite wheel N is 

 added to strengthen the connection between the two drums, and 

 to relieve all the spindles from pressure. Before leaving the 



