126 
HEATING HOT-HOUSES BY HOTWATER. 
The density of the fluid in the vessel A, will also decrease in con¬ 
sequence 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 up¬ 
per 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 obvi¬ 
ously continue till the temperature be nearly the same in both ves¬ 
sels, 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. 
14 
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 temperature 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. 
It will be evident to any person of philosophical research, how¬ 
ever, that in considering water the only liquid capable of being em¬ 
ployed, 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 boil¬ 
ing points of different liquids are at such different temperatures that 
we may vary the ultimate temperature of the healing surface from 
100 to 600 degrees, that of water being 212 degrees. This mode of 
considering the subject, opens a new source of speculation and im¬ 
provement. 
