250 HEATING AS APPLIED IN HORTICULTURE. 



who will not progress with the times. 

 Hot water, as a heating medium, has had 

 that opposition to contend with, and every 

 little accident, to which all improvements 

 are for a time liable, has been taken hold 

 of. A boiler will not heat so soon as 

 expected. How many common flues will 

 not draw, let us ask, because the furnaces 

 have been improperly built 1 The boiler 

 may heat, but the water refuses to circu- 

 late. What is the cause of this? Only two 

 can be stated— namely, the pipes being 

 laid lower in one part than in another — 

 that is, out of a direct line as regards 

 their level. But this seldom occurs. By 

 far the most frequent cause of a want of 

 circulation is air getting in — as, when 

 water is boiled for the first time, the air 

 it contains is expelled; and this goes on 

 accumulating, until the pipes at some par- 

 ticular part become fully charged with it 

 instead of water. The very process of 

 filling the apparatus tends to produce 

 this effect; for as water is poured into the 

 boiler, which, being always the lowest 

 point, will of course be filled first — and 

 as the lower pipe will next receive the 

 water, and afterwards the top one — it fol- 

 lows that the air which is in the pipes, 

 unable to find its way out through the 

 water in the boiler, is forced into some 

 part of the pipe generally at the highest 

 point, and there remains compressed 

 between the two bodies of water, and 

 consequently separating them. Till this 

 air be extracted, no circulation can take 

 place. In the case of close-topped boilers, 

 accidents might occur attended with dan- 

 ger. To remedy this, a small perforation 

 should be made in the pipe at the highest 

 point, about the size of a quill ; " for," 

 as Hood remarks, " the rapidity of motion 

 in fluids being inversely proportional to 

 their specific gravities, as water is 827 

 times more dense than air, an aperture 

 which is sufficiently large to empty a 

 pipe in fourteen minutes if it contained 

 water, would, if it contained air, empty 

 it in about one second." 



Some have recommended a self-acting 

 valve, others a cock, and some square 

 openings fitted with corresponding stop- 

 pers. The usual plan adopted by our- 

 selves is a simple aperture about one 

 quarter of an inch in diameter, having 

 a wooden peg loosely fitted into it, so that 



if the pressure of the air be considerable, 

 it will force the peg out. Where it can 

 be applied conveniently, we insert into 

 the upper part of the flow-pipe a small 

 tube of several feet in length, and fix it 

 to the wall, leaving the orifice always 

 open. 



"The velocity with which the water 

 circulates has been attempted to be given 

 by calculating from the difference of tem- 

 perature indicated at opposite points of 

 the apparatus. But many circumstances 

 in practice affect the motion ; — varying 

 temperature of the air, and consequently 

 of the pipe surface — the flexures, and in- 

 clinations, and length of the pipes — the 

 varying rate of combustion in the fur- 

 nace, and heat of the water in the boiler — 

 the thickness, material, and surface of the 

 pipe — its irregular or uniform calibre — 

 the greater or less skill with which the 

 air- valves are placed, and many mechani- 

 cal niceties of detail and finish. Most of 

 the data must be assumed, so that, to 

 practical men, the few rules that have 

 been given are not only useless, but often 

 delusive ; and hence the uncertainty and 

 varying practice of those who have 

 erected this sort of apparatus. Some 

 general maxims are, however, recognised 

 in these arrangements. As the effect of 

 the circulating-pipe depends on the tem- 

 perature of its surface, that will be higher 

 or lower as the velocity of the internal 

 fluid is rapid or slow. All bends or 

 flexures in the pipe should be avoided ; 

 and, where unavoidable, they should have 

 as flat a curve as possible. Angular 

 elbows or knees are most faulty. The 

 ascending and descending pipes should 

 throughout have the same, or nearly the 

 same, internal diameter, without contrac- 

 tions or enlargements ; and where cocks 

 are introduced, their water-way should 

 be as near that of the pipe as possible. 

 It is obvious that, in a series of tubes in 

 which the circulation depends on a mi- 

 nute difference of level, this should be 

 carefully preserved by proper fixing, more 

 particularly where the direction is nearly 

 horizontal. A slight accidental depres- 

 sion in a series of pipes will mar the 

 action of an otherwise effective appara- 

 tus ; and more failures have arisen from 

 this than by all other causes united." — 

 Beknan's History of Heating, &c. 



