APPLICATION OF HOT WATER IN HEATING HOT-HOUSES. 
101 
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 
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 efl^ect ; 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, 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 of diff'erent liquids are at such diff'erent temperatures that we may vary the 
ultimate temperature of the heating 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 improvement. 
Fig.. 2. 
From the common principles of hydrostatics and the equations obtained, the 
following practical deductions may be derived : — 
1st. The more expansible the liquid is, by a given change of temperature, the 
greater will be the velocity. 
2ndly. All other things being the same, the velocity will be increased in 
proportion to the square root of the depth of the boiler ; therefore, in a boiler four 
times as deep the velocity will be doubled. 
3rdly. If there be a suflScient service of pipe for the object required, a reservoir 
is not necessary to the motion of the water : a simple bent pipe, fig. 2, being all 
that is essential to motion ; the reservoir is only to reserve a hot mass of water to 
maintain the heat after the fire has gone out. 
4thly. If a boiler have suflScient surface to receive the eflfect of the fire, and 
the whole apparatus contain as much water as will convey the heat from the fire 
to the heating surface in the time corresponding to its velocity, its capacity need 
