PHYSICAL PROPERTIES OF WATER, ETC. 



21 



to the same pressure (that of one mass of liquid), and the total thrust on each is, of 

 course, proportional to its section. In the French instrument the pistons are subjected 

 to equal total thrusts, being exposed respectively to fluid pressures which are inversely 

 proportional to their sections. The British instrument is employed for the purpose of 

 overcoming great resistances by means of moderate forces ; the French, for that of 

 measuring great pressures in terms of small and easily measurable pressures. 



Amagat's notable improvement consists in dispensing with the membrane, or sheet 

 of india-rubber, which was one of the main features of the old Desgoffes manometer, 

 and making his large, as well as his small, piston, fit all but tightly the hollow cylinders 

 in which they play : — a very thin layer of viscous fluid passing with extreme slowness 

 between each piston and its cylinder. The adjustment is very prompt, even in winter 

 when the viscosity of the fluids is greatest : — but it is made almost instantaneous by a 

 simple but ingenious device, which enables the operator to give the pistons a simul- 

 taneous motion of rotation. The following directions which accompanied the instrument 

 will enable the reader fully to understand its construction and use. I have given an 

 accurate version, not a literal translation, of them : — 



"Process of setting up the Apparatus. 



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