90 MR RUSSELL'S RESEARCHES IN HYDORDYNAMICS. 



graduated in decimals of an inch, were inserted, as Tj, Tj, T3, T4, T5, Te, Plate III. 

 Fig. 5, into which the external fluid was pressed up to the level of the surface 

 of the quiescent fluid. The action of these gauges was found veiy delicate, a 

 slight variation in the position of the tube, or a trivial error in the formation of 

 its aperture at the bottom, giving irregular results. The value of the indications of 

 the tubes was determined by drawing the vessel in opposite du-ections, which im- 

 mediately shewed which of the tubes were affected by errors of position : — those 

 which were free from such errors were selected for observation, and their indica- 

 tions are given in the following experiments. 



(3.) Dynamometers. — Much has been written on the subject of dynamometers, 

 and much in praise of a species of that instrument, in which the minor oscillations 

 of the moving force, or the variations of the resistance, are suppressed, and only some 

 unloiown function of these variations supposed to approximate to their sum, or ra- 

 ther their mean is exhibited, this effect being produced by the application of the well- 

 known principle of the retardation of a fluid passed through a very small aperture. 

 I made trial of a very simple dynamometer formed on this principle, which was very 

 accurately constructed for me by Mr John Adie. A helical spring contained in 

 a cylinder, was compressed by a piston, which communicated through the piston- 

 rod with the moving power. The cylinder being closed was filled with oil, and a 

 communication between opposite ends of the cylinder effected through an exter- 

 nal tube, governed by a stopcock. The stopcock gave the means of retarding or 

 facilitating the passage of the fluid, and enabled the observer to render the posi- 

 tion of the index more or less stable, by turning the stopcock in such a way as to 

 facilitate or retard the motion of the fluid in the variations of force. I had at 

 first considerable faith in this species of instrument. It certainly accomplished 

 the purpose of giving a stable instead of an oscillatory indication, an indication 

 easily observed. But it may be questioned, whether it be really a desideratum to 

 obtain indications which have not the variations of the subjects themselves that 

 are to be measured. The indications of the instrument are in truth false, or at 

 least they only shew what effect the action of a desultory force produces on the 

 motion of the fluid of the instrument itself. In applying this instrument to the 

 measurement of the resistance of fluids, when the resistance is by no means very 

 desultor}'-, it is most desirable that the variations of the power should be apparent, 

 instead of being rendered latent. It is obvious that the force communicated by 

 jolts to a body in motion, produces effects that are so widely different from those 

 of uniform pressure, that the sum of the impulses due to a given velocity is very 

 different in its effect from a uniform pressure equal to that sum. 



The disadvantages of using a desultory power like that of horses in producing 

 motion, to which the resistance is like that of a fluid continuous, are very great. 

 The following examples at velocities almost precisely equal, made with the same 



