190 



HEATING AS APPLIED IN HORTICULTURE. 



winding channels ; and also, by the 

 obstructions and hindrances caused by the 

 angles projecting over the passages, to 

 cause fresh particles of air continually to 

 impinge on the surface. There does not, 

 however, appear to be much gained prac- 

 tically by compelling the air to pursue 

 this tortuous course. What is gained in 

 temperature will be lost in time. It has 

 been objected to these modes of increasing 

 heating surfaces by projections, that there 

 will be diminution of efficiency from the 

 ends of the projections not being in such 

 close contact with the steam, water, or 

 other vehicle of heat that may be em- 

 ployed, as the sides of the plates. But 

 within proper and moderate limits as to 

 the length of the projections, this objec- 

 tion has not been found to hold good in 

 practice, which will be more obvious 

 when we consider that iron is one of 

 the best conductors of heat, while air 

 is a very slow absorbent ; and conse- 

 quently, that for some limited distance 

 heat will travel through the metal more 

 rapidly than the air can take it up, unless 

 blown through the hot chamber with 

 great rapidity." — Walker's Hints on Ven- 

 tilation. 



" Whatever be the arrangements 

 adopted for heating by hot water, two 

 considerations must be specially attended 

 to, namely, sufficient strength to bear the 

 hydrostatic pressure, and freedom of 

 motion for currents of water of varying 

 temperatures, and consequently of vary- 

 ing densities. As fluids transmit their 

 pressure equally in every direction, a 

 column of water rising from a strong 

 vessel to a certain height may be made 

 to burst the vessel with enormous force. 

 Thus a tube, whose sectional area is 1 

 inch, rising to the height of 34^ feet from 

 the bottom of a vessel of water, will, if 

 the tube be also full of water, exert a 

 bursting pressure on every square inch of 

 the inner surface of such vessel of one atmo- 

 sphere, or 1 5 lb. If the sectional area of the 

 tube be increased, the pressure remains 

 the same. If a boiler be 3 feet long, 2 feet 

 wide, and 2 feet deep, with a pipe 28 feet 

 high from the top of the boiler, when the 

 apparatus is filled with water there will 

 be a pressure on the boiler of 66,816 lb., 

 or very nearly 30 tons. This will show 

 the necessity for great strength in the 

 boiler, especially when it is considered 



that the effect of heat upon it is to dimi- 

 nish the cohesive force of its particles. 

 But even supposing the apparatus were 

 to burst, no danger would arise, because 

 water, unlike steam, has but a very limited 

 range of elasticity. The boiler just de- 

 scribed would contain about 75 gallons 

 of water, which, under a pressure of one 

 atmosphere on the square inch, would be 

 compressed about 1 cubic inch ; and if 

 the apparatus were to burst, the expan- 

 sion would only be 1 cubic inch, and the 

 only effect of bursting would be a crack- 

 ing in some part of the boiler, occasioning 

 a leakage of the water." 



Heating by hot-water pipes and tanks 

 or gutters, in all their modifications, is a 

 most decided improvement on the old, 

 unsteady, and expensive mode of heating 

 by means of fermenting materials. There 

 is, however, one advantage peculiar to the 

 old mode of heating, namely, the supply 

 of ammonia and other gases to the plants 

 from the stable dung during the process 

 of fermentation. This is also to be at- 

 tained, as recent experiments have proved, 

 by the introduction of liquid manures of 

 the most powerful kinds, in various ways, 

 in structures heated by the new method. 

 Mr Fleming of Trentham, by mixing 

 pigeon's dung with the water in the tanks, 

 in the proportion of 1 ounce of dung to 

 1 gallon of water, has obtained a supply 

 of this invigorating gas, which seems to 

 be so essential to vegetable life, and pro- 

 bably others also. It is, and has long 

 been known to the best cultivators, that 

 plants of all kinds thrive better in pits 

 heated by fermenting manure than any- 

 where else. Now, this increase of vigour 

 is well known not to arise from any dif- 

 ference in the humidity of temperature 

 that may exist between the two methods 

 of heating, but from a deficiency of am- 

 monial gas in the one case, and an abun- 

 dance of it in the other. We apprehend 

 that by mixing the manure with the 

 water, as in the case alluded to, in the 

 tanks or pipes, the insoluble parts will 

 find their way into the boiler, and hence 

 cause the formation of incrustations 

 in them, which tend so much to their 

 decay. We adopt a different course to at- 

 tain the same end, namely, by watering the 

 beds and floors over the tanks with urine, 

 or rich liquid manure, and also by plac- 

 ing shallow vessels, kept filled with the 



