274 



NATURE 



{Augtist 2, 1877 



plane or disc of water, rotating in its own plane between the 

 diaphragms, which define the direction of the water disc and 

 which are the boundaries of its thickness. 



Having now traced the modus cperanai by which the reaction 

 is produced, it is necessary to show that (i) an adequate amount 



of total reaction can be produced by an instrument of conveniently 

 limited dimensions ; and that (i\ an instrument of given dimen- 

 sions is governable as regards its reactions, that is to say, is 

 capable of being made to produce at pleasure a greater or less 

 reaction with a given number of revolutions, so that vrithin 



Enlarged 



of dynamometer attached to screw shaft. 



reasonable limits the same instrument shall be capable of dealing j in the appendix, that comparing two strictly similar but differ- 

 with engines of great or small power, allowing each to make its ently dimensioned instruments, their respective "moments of 

 proper number of revolutions. reaction " with the same speed of rotation in each, should be as 



As regards condition No. I, the theory shows, as will appear | the fifth powers of their respective dimensions. 



of dynamometer attached 1 



This proposition is fully borne out by experiment. Mr. Froude 1 at a given speed of turbine rotation should also have been 4. 



has had a pair of similar instiumenls made, in which the turbine The ratio was in fact 3S6 ; but the small difference is referable 



diameters are respectively 12 in. and O'l in Now^il V=4 to the ciictmstance that in the larger of the two instiuments the 



^ Vq'I/ mtemal surlace was rather less smooth and the inction of the 



and accordingly the ratio of the moments of the twolinstruments ) water consequently rather greater than in the^ other. The data 



