308 



FORCING GARDEN. 



parts of the vessels, and the other the upper parts. 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 

 b b. 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 fluid above the centre of the upper pipe is 

 of greater weight than the column, /<?, 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 



