270 



HEATING AS APPLIED IN HORTICULTURE. 



promoted by what is technically called a 

 return or descending pipe. But in many 

 cases in which the difference of level is 

 small, the effect does not seem to have 

 been impaired by this arrangement. In 

 the diagram, fig. 358, a is a furnace 



Fig. 358. 



e 



in the inside of the boiler s, with vari- 

 ous pipes proceeding from it. If we 

 suppose the pipe e only to be attached 

 to it, and to be filled with water, as 

 that in the boiler was heated, a circu- 

 lation would commence and continue 

 by the hottest water rising to the upper 

 part of the pipe, and the water cooled by 

 contact with the surface falling into the 

 boiler. It is clear the same effect will be 

 produced if the pipe i were substituted 

 for the pipe e ; and it is also clear that 

 the circulation would be less impeded 

 than if the hottest water rose to i, and 

 descended, when cooled, through the 

 pipe n nto the bottom of the boiler s — 

 supposing in both cases an equal quanti- 

 ty of heat to be dissipated. If the heat- 

 ing pipe o was nearly horizontal, the same 

 effect would take place from the mole- 

 cular action : the water in the pipe would 

 be somewhat warmer than if it flowed 

 along b and descended by d, and return- 

 ed by c into the boiler, as less of the 

 velocity due to the temperature would be 

 lost from friction in the straight pipe 

 than in the return-pipe. In most cases 

 the return-pipe might be altogether 

 omitted, with manifest advantage to the 

 simplification, and consequent certainty, 

 of the circulating process. The lightest 

 water will always find the highest level ; 

 and the less it is impeded, the circulation 



will be the more perfect, and the heating 

 effect will be greater." 



The following is Mr Tomlinson's theory 

 of circulation : — " When heat is applied 

 to a vessel containing water, the principle 

 of conduction altogether fails ; for water 

 is so imperfect a conductor of heat, that 

 if the fire be applied at the top, the water 

 may be made to boil there without greatly 

 affecting the temperature below. But when 

 the fire is applied below, the particles in 

 contact with the bottom of the boiler, being 

 first affected by the heat, expand, and 

 thus, becoming specifically lighter than 

 the surrounding particles, ascend; and 

 other particles take their place, which, 

 in like manner becoming heated, ascend 

 also ; — and the process goes on in this 

 way until the whole contents of the boiler 

 have received an accession of tem- 

 perature. If the process be continued 

 long enough, the water will boil and pass 

 off in steam ; if the boiler be closed in on 

 all sides, so as to prevent the escape of 

 steam, it will burst with a fearful explo- 

 sion. If a tube full of water rise from 

 the top of the boiler in a vertical line to 

 any required height, and then by a series 

 of gentle curves descend, and enter near 

 the bottom of the boiler, the process of 

 heating is still the same. The particles 

 of water first heated will rise, and, in 

 doing so, distribute their heat to other 

 particles, which will also rise; these in 

 their turn will lose a portion of their heat 

 to other particles, which rise in their 

 turn, until at length an equilibrium is 

 established. But as the source of heat is 

 permanent, other particles are rapidly 

 brought under its action, and, being 

 heated, ascend. By continuing the pro- 

 cess a short time, the particles in the 

 vertical tube become heated, and, by 

 their expansion, exert a pressure on the 

 water contained in the lateral branches; — 

 this, together with the increasing levity 

 of the water in the boiler, establishes a 

 current, and the water from the branches 

 begins to set in in the direction of the 

 boiler ; the water in the lowest branch, 

 where it enters the boiler, supplying 

 colder and heavier particles every mo- 

 ment, to take the place of the warmer and 

 lighter particles which are being urged 

 upwards along the vertical pipe." 



