308 FORCING GARDEN. 
parts of the vessels, and the other the upperparts, If the 
vessels and pipes be filled with water, and heat be applied 
to the vessel A, the effect of heat will be to expand the 
water in the vessel A; and its surface will, in consequence, 
rise to a higher level,-a a, the former general level being 
66. The density of the fluid in the vessel A will also de- 
crease, in consequence of its expansion; but as soon as the 
column, c d, of fiuid above the centre of the upper pipe is 
of greater weight than the column, fe, 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, the motion will obviously 
continue till the temperature be nearly the same in both 
vessels ; or if water be made to boil in A, it may also be 
boiling hot in B, because ebullition in A will assist the 
motion.” 
The figure referred to in the preceding quotation, repre- 
senting the common tank boiler surrounded by a flue with 
a cistern at the extremity of the pipes, exhibits the form 
in which the apparatus was first erected; but as in this 
Fig. 29. 
arrangement the process of heating was very slow, many 
