280 THE SCIENTIFIC PAPERS OP 



the water, by partaking fully of its onward motion, without 

 sensibly impeding or agitating the same. 



These conditions are most fully complied with, by a perfect 

 screw suspended on two pivots, in the axis of the moving column 

 of water. They are also fulfilled by a Barker's-mill, or turbine of 

 spiral blades, that yield to the motion of the water outward from a 

 centre. 



If, on the other hand, the vanes of the propeller are of 

 irregular shape, so as to form eddies or obstructions in the 

 water, it will be theoretically impossible to insure a uniform 

 increasing rate of rotation with increased velocity of current ; 

 for the retarding or accelerating effects produced by eddies or 

 concussions increase not in the simple, but in the square ratio 

 with the velocity. 



The correctness of this argument was proved indirectly, and 

 unknown to the writer, by the failure of an attempt made at about 

 the period referred to, by Mr. Abraham, to register the water 

 flowing through a pipe by means of a screw propeller of irregular 

 form, although suspended with great care between points of agate. 



The same unsatisfactory result was obtained some years later by 

 Mr. Tebay, of London, who formed his propeller by making radial 

 incisions into a disc of brass plate, mounted upon a spindle, and by 

 twisting each segment in the same manner, like the vanes of a 

 windmill. He endeavoured to counteract the inaccuracy of his 

 propeller, by introducing valves so contrived that the water should 

 be able to pass only at a fixed velocity. 



In order to obtain correct measurement by an " impact meter," 

 it is not sufficient that the propeller should yield equally in all its 

 parts to the motion of the water, but it must also 1 possess the 

 power to overcome a uniform resistance by friction in its bearings, 

 &c., without diminishing its proportionate rate of rotation at low 

 speeds. 



The apprehension of these difficulties deterred the writer, for 

 several years, from proceeding, until the pressing want for a meter 

 to carry out some other improvements induced him to construct, 

 in 1850, the identical meter now before the meeting ; and which, 

 in point of accuracy of measurement and compactness, fully 

 satisfied a committee of inquiry of the Manchester Corporation 

 Water Works, by whom its adoption was recommended. The 



