January 20, 1870. ] 



JOURNAL OF HORTICULTURE AND COTTAGE GARDENER. 



45 



forated places. We have tried a little in the same way, but we 

 would prefer one opening, at the farther end of the piping 

 from the chamber. You say nothing of the heated air being 

 fresh. This you could secure by taking a small pipe to the 

 bottom of the chamber from the external air. Without perfo- 

 rations, if the pipe become hot, it would heat the air near it 

 and thus prevent a stagnant atmosphere. You may moisten 

 this heated air as it comes out of the chamber. If you depend 

 merely on tne air of the house there is one suggestion more we 

 would make — take vour heating pipe from as near the top of 

 the chamber as possible, and have an opening nearly as large 

 close to the bottom, that cold air may euter as the heated air 

 goes out. If you use perforated pipes, be so kind as to let us 

 know the results.] 



BOILERS AND BOILING. 



I did not expect to convert Mr. Woolfield to my own views, 

 and, therefore, am not surprised that he does not agree with 

 my reasoning. I think, however, the discussion of the subject 

 is likely to do good, as many persons have very mistaken 

 notions as to the motive power in heating by an ordinary hot- 

 water apparatus, and I am glad that Mr. Woolfield agrees with 

 me in thinking it well to ventilate the subject. 



To return once more to the question as to heat applied at 

 the top of a boiler, I do not rely in the least on conduction 

 from one particle of water to another. I know the conducting 

 power of water to be very small, but your readers must bear in 

 mind that the water in a heating apparatus is in constant cir- 

 culation, and when in motion the particles of water are con- 

 stantly changing position ; it is not true, then, that only a 

 small layer of water is acted upon by the upper surface of a 

 saddle boiler, because a fresh current is constantly supplied, 

 and the heat is constantly acting on fresh particles. The law 

 of conduction of heat from one particle of water to another 

 hardly comes into play at all, as I consider the water is heated 

 by actual contact with the heated sides of the boiler, and there- 

 fore by conduction from the iron itself. Mr. Woolfield has 

 mistaken my meaning in thinking I meant that the laws of 

 conduction, convection, or radiation were different in one case 

 to another. I may not have expressed my meaning carefully 

 enough ; I only meant that the conditions under which the 

 water is heated are altered. In an ordinary boiler by the side 

 of a fire the water is in a state of rest, and if heat is applied by 

 a flue on the upper surface it has to act through a stratum of 

 air, unless the boiler is sufficiently full for the water to touch 

 the top, in which case any heat from the surface would act by 

 conduction alone, and would be very inefficient, but still would 

 have more effect than when the boiler was only partially filled. 

 In a steam boiler heat applied to the surface of the boiler has 

 to act through a stratum of steam, which is a much worse 

 conductor of heat than water, and little better than air ; and I 

 have no doubt it is true that if a cylindrical boiler for gene- 

 rating steam were entirety surrounded by fire it would evaporate 

 very little more than if only the under surface were exposed to 

 the action. The heat applied at the top of the boiler, there- 

 fore, in the three cases acts — in the horticultural boiler directly 

 on the water in motion by direct conduction from the iron ; in 

 the second through a stratum of air, which is a very bad con- 

 ductor of heat ; in the third through steam. 



Again, I do not consider that water in a horticultural boiler 

 is heated by convection in the same way as water in a boiler 

 by a kitchen fire or a kettle on the fire top, because in the case 

 of the kettle the heated water from the bottom and sides rises, 

 and cold water falls t) take its place, till all is heated to the 

 game degree. In a horticultural boiler the heated water, as I 

 pointed out in my last, rises directly and passes off up the flow 

 pipe, cold water being supplied by the return pipe from below ; 

 and as, in an ordinary saddle boiler, the sides and top seldom 

 contain more than a 3{-inch stratum of water in a constant 

 state of motion, I do not think that water ever descends from 

 above as in a kettle to supply the place of heated particles of 

 water that are rising. It is my belief, which Mr. Woolfield has 

 not at all shaken, that the water in a horticultural boiler is 

 almost entirely heated by direct conduction from the iron — a 

 first-class conductor of heat — and not by convection or conduc- 

 tion of heat from one portion or particle of water to another ; 

 and I am still of opinion that the heated exterior surfaces of a 

 boiler have nearly as much effect as the interior. 



The argument from the box 6 feet high and 1 foot square 

 heated from above or below has very little to do with the case. 

 I am perfectly aware that only the upper stratum of water 



would be heated, or at all events that it would take a long time 

 for the water to be much heated by conduction if the fire were 

 applied in such a case as this to the top only ; besides which, 

 as there could be no current of air to the bottom of the fire, 

 the heat from the fire itself would be very feeble. I have had 

 this argument brought before me often in talking the matter 

 oeer, and I rather expected it would be brought before me 

 again. I simply answer, it does not apply in the least to hor- 

 ticultural boilers, but it does to steam and ordinary boilers. 

 Mr. Woolfield rather begs the question when he says all scien- 

 tific men agree with him, and tries to put me in the invidious 

 position of asserting my own individual opinion in the face of 

 science. All I endeavour to maintain is, that there is a pre- 

 conceived notion, contrary to scientific facts, that the heat 

 applied from above in horticultural boilers is of no use, nearly 

 all experiments having been made on water in a state of equi- 

 librium and not in motion. 



If the motion of water in a small pipe is as free from friction 

 as in a large pipe, all I can say is, that the laws of hydrostatics 

 are changed since I was at college some twenty years ago. 



Next about the comparative advantages of 4 and 2-inch pipes. 

 It will take 100Q feet of 2-inch pipes to do the work of 500 feet 

 of 4-inch pipes, as far as the radiation of heat is concerned. The 

 water in every foot of the 2-inch pipes is cooled down by radi- 

 ation twice as fast as the water in the 4-inch pipes in giving 

 out the same amount of heat, and therefore the whole water in 

 the 1000 feet of 2-inch pipe is cooled down four times as much 

 as in the 4-inch pipe. I am aware that the 500 feet of 4-inch 

 pipe will contain twice as much water, but I maintain that the 

 water after traversing 1000 feet of 2-inch pipe returns to the 

 boiler much colder than it does through 500 feet of 4-inch pipe ; 

 and though the rapidity of the flow is increased, yet the water 

 remains a shorter time in the boiler, and as it returns colder 

 it requires a greater amount of fire to keep up the heat, though 

 it will be more quickly heated in the first instance, and for 

 night work there can be no comparison in the efficiency of a 

 2-inch and a 4-inch pipe. 



Now. anyone who has very much to do with boiler fires must 

 know that when it is necessary to keep up a bright fire there 

 is much more waste of heat through the heated gases passing 

 up the chimney, and I am certain that it will require more 

 attention to the fire, and a greater consumption of fuel, to keep 

 up the heat in a boiler when heat is radiated off through 

 1000 feet of 2-inch pipe than it will to keep up the heat when 

 it is radiated off through 500 feet of 4-inch pipe. The specific 

 heat of water — that is, its capability of containing heat, is 

 greater than that of any known substance ; the specific heat, 

 therefore, of 500 feet of 4-inch pipe is double that of 1000 feet 

 of 2-inch piping. Water parts with its heat slowly, but gives 

 off its heat more rapidly when at a high temperature than 

 when at a lower point ; as, therefore, the water in passing 

 through 2 inch pipes gives off its heat, it becomes less effi- 

 cacious in heatiDg a house according to the length of pipe 

 which it has traversed. So that I cannot see any one advan- 

 tage that a 2-inch pipe has over a 4-inch one ; and the larger 

 the quantity of pipe there is to deal with the greater is the 

 disadvantage of the 2-inch over the 4-inch pipe. A cubic foot 

 of water at 60° weighs about 61 lbs. 10 ozs. ; when heated to 

 boiling point it weighs about 59 lbs. 5 ozs. But water gets 

 more rapidly light the nearer it approaches the boiling point : 

 Thus 1000 parts of water at boiling point when cooled 36° F. 

 below 212' become 986, at 68° 960. Now, as the difference of 

 weight between cold water and heated water represents the 

 force of the current or motive power in pipes, and as 12 feet of 

 4-inch pipes contain about a cubic foot of water, while it takes 

 48 feet nearly of 2-inch pipe to contain the same amount, I 

 consider the motive power in the larger pipe to be the greater. 



Mr. Woolfield says with regard to the radiation of heat from 

 glass in windy weather, it is better to take into account the cold 

 j air entering by crevices, and provide more pipe accordingly. 

 ' I consider the air entering through the laps of glass has very 

 little to do with the question. It is a very common expression 

 to speak of cold striking through, as if cold were a definite 

 thing of itself ; but I need hardly say that cold is an indefinite 

 term, and when we speak of one thing being colder than 

 another it merely means that it contains less heat. Now, glass 

 houses are cooled down chiefly by radiation — that is, the heat 

 inside is given off to heat the air in contact with the glass 

 outside, and it is here where wind plays an important part. 

 Air is a very bad conductor of heat, but heat radiates very 

 rapidly through it, and on a windy night a fresh layer of colder 

 air is constantly coming in contact with the glass and takes 



