324 



FORCING GARDEN. 



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 h h. 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 upi^er pij^e is of 

 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 equili- 

 brium of the fluid will cause a corre^onding 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, re^ 

 presenting 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 



Fig. 29. 



this arrangement the process of heating was very slow, 

 many changes have been made ; the cistern has gene- 



