570 Application of Hot Water in Heating Hot-houses. 



being b b. The density of the fluid in the vessel A, will also 

 decrease in consequence of its expansion, but as soon as 

 the column c d of fluid above the centre of the upper pipe 

 is of a greater weight than the column / e above that centre, 

 motion will commence along the upper pipe from A to B, 

 and the change this motion produces in the equilibrium of 

 the fluid will cause a corresponding motion in the lower pipe 

 from B to A ; and, in short pipes the motion will obviously 

 continue till the temperature be nearly the same in both 

 vessels, or if the water be made to boil in A, it may also be 

 boiling hot in B, because ebullition in A will assist the 

 motion. 



5. The causes which tend to retard the motion of water 

 in the pipes are, 1st, the contraction of the moving fluid at 

 the orifice of the pipes ; 2nd, the friction of the fluid in the 

 pipes, which sets the limit to the distance to which the 

 pipes can be extended to produce the proper quantity of 

 useful effect; but, it is remarkable that the higher the tem- 

 perature of the moving fluid the less its friction ; 3rd, the 

 motion is retarded by the cooling of the fluid in its progress 

 along the pipes, such cooling having a tendency to produce 

 a double current ; and 4th, by bends and changes of form. 



6. It will be evident to any person of philosophical re- 

 search, however, that in considering water the only liquid 

 capable of being employed, we should be losing sight of one 

 of the greatest advantages resulting from the knowledge of 

 natural phenomena, for all liquids expand by heat; and 

 hence, in all of them its partial application would produce 

 motion under proper circumstances ; while the boiling points 



