94 G. H. KNIBBS. 
height to which the liquid will rise in tubes attached normally to” 
the surface of the pipe,! the connecting orifices being very small _ 
in relation to the sectional area of the flow. Such an arrange 
ment cannot be realized with capillary tubes so that the fall in 
pressure must be deduced from measures of the pressures in the — 
reservoirs supplying and receiving the flowing liquid. The sectional 
areas of these should be, and will be assumed to be, very large in 
relation to the sectional area of the capillary. 3 
In Fig. 1, in which A denotes the supplying and B the receiving 
reservoir—i.e. the flow being supposed to take place in'the direc 
tion A B—the points A and Blie in the axis of the tube ZH’ FP, 
supposed horizontal ; to these points the measures of the pressures 
in the reservoirs are referred, P being the difference of those 
pressures. It is immediately evident from the principle of col 
servation of energy, that, neglecting losses from frictional resis 
tances, the pressure at the section ee’ where the régime of 
parallelism of flow is established, is less than the pressure in the 
reservoir A, owing to the change from potential to kinetic energy 
in the effluent liquid. This was not overlooked either by J acobson 
or by Hagenbach as already pointed out. The latter divided 
the head, or charge as it is often called, as measured by the 
difference of pressures, and expended in overcoming resistance 
into two' parts, the velocity-head (Geschwindigkeitshéhe) and the 
head due to frictional resistance within the tube (Widerstandhihe). 
Hagenbach’s formula? may be written, if we adopt an absolute 
notation, or C.G.S. units? } 
1 This is a called the piezometric measurement of the prewar 
by hydraulici 
2 Pogg. ‘ear Bd. 109, p. 408, 1860. 
* Using C.G.8. units we shall express P in dynes per square centime 
thus P= pg, if p be the pressure in grammes per square centimetre, and 
the unit o ntly 
in dessin the prunmaea the portalatios ¢ tiles of | the viscosity 7 me 
erred to the above by the equation 7’ = yig- The absence of the § 
term in the quotation of Hagenbach’s equation is thus explained. 
