264 HEATING AS APPLIED IN HORTICULTURE. 



or returning pipe being of greater specific 

 gravity than that in the boiler ; and that 

 motion takes place in the lower pipe first, 

 instead of in the higher one, as was for- 

 merly supposed. The colder the water is 

 in the lower pipe when it enters into the 

 boiler, the more rapid will the circulation 

 be ; and as it is desirable to have, in most 

 cases, a rapid circulation, it follows that 

 the greater length of pipe employed, the 

 more likely is this to be effected ; because 

 the greater the length of pipe, the more 

 surface is produced for radiation and 

 conduction. Hence four courses of pipes, 

 as in the annexed cut, fig. 346, will give 

 out more heat than two ; and the specific 



Fig. 346. 



gravity of the fourth, on entering the 

 boiler, will be much greater than at the 

 same end of the second pipe. The water, 

 having given out nearly all its heat in 

 the course of a long circuit, must natur- 

 ally absorb more heat from the fire than 

 if it had made a shorter circuit, and 

 entered the boiler in a much less cold 

 state — consequently economising fuel. 

 We have often been struck in practice by 

 finding the same quantity of fuel used in 

 heating the atmosphere of a hothouse, by 

 the pipes running parallel to the front 

 wall, as is usually the case, as was requi- 

 site when the water was turned into pipes 

 or tanks under the beds to supply bottom 

 heat also, — a consequence arising from an 

 insufficiency in the extent of pipes, and 

 a circumstance too little attended to. 



It appears by Hood's experiments, that 

 another erroneous opinion has been enter- 

 tained regarding an increased circulation 

 being caused by giving the return-pipe a 

 considerable fall towards the boiler. This, 

 so far as regards boilers that are open at the 

 top, and adapted to the original or hori- 

 zontal circulation, is found, he says, to be 

 erroneous ; and although advocated by 

 Tredgold and other scientific men, who 

 founded their theory upon consideration 

 of the subject as a simple question of 



hydraulics, instead of a compound result 

 of hydrodynamics, has been proved by 

 Hood and others to be attended with a 

 loss on the effective pressure rather than 

 a gain, as the height between the highest 

 part of the pipe and the boiler is exactly 

 the same whether the pipes be laid verti- 

 cally or inclined. 



In the arrangement made by Rogers 

 as to the position of his pipes to that of 

 the boiler, he recommends that, when 2 

 or 3-inch pipes are used, a uniform rise 

 of about 1 inch in 20 feet should be 

 given them ; but when 4-inch pipes are 

 employed, it is better that the pipes 

 should fall from the boiler in exactly the 

 same proportion, if local circumstances 

 will admit of the arrangement ; for, he 

 says, " the moving force, and consequently 

 the velocity of the circulation, depend 

 upon the difference in weight between the 

 ascending and descending columns : now, 

 the greater the height of these columns, 

 the greater the difference in their weight, 

 and consequently the greater the velo- 

 city of circulation, and the higher the 

 mean temperature of the pipes. With 

 4-inch pipes this is unimportant, for the 

 volume of water contained in them is 

 large as compared with their radiating 

 surface ; so that, with any given velocity, 

 the water loses less heat in one circula- 

 tion than it would in smaller pipes. 

 Moreover, in 4-inch pipes the fric- 

 tion is small, and offers little resistance 

 to circulation, so that in 4-inch pipes 

 there is little need of very rapid circula- 

 tion, and less resistance to such circula- 

 tion ; but with smaller pipes there is 

 need of more rapid motion, or the water 

 in the return-pipe will be much colder 

 than in the delivery- pipe. To take the 

 instance of 2-inch pipes, and suppose the 

 circuit of equal length, and the velocity 

 equal in both cases, the difference of tem- 

 perature between the flow and return 

 pipes will be four times as great as it 

 would be with a 4-inch pipe, because the 

 volume of water conveying heat is only 

 one-fourth of that contained in the larger 

 pipes. It is true, this difference of tem- 

 perature increases the difference of weight 

 in the column, — that is, the moving force 

 — and the supposition of equal velocities 

 does not hold good in practice. But with 

 increased velocity comes increased fric- 

 tion ; and, moreover, with diminished 



