268 



HEATING AS APPLIED IN HORTICULTURE. 



particularly such as are close-topped. 

 This we consider a very minor affair, and 

 prefer such as have a gauge-pipe and 

 supply-pipe communicating with some 

 convenient spot near the stoke-holes, the 

 former furnished with a cock, the latter 

 left open to serve as an expansion tube. 

 Two or three times a-week these are ex- 

 amined, the cock of the gauge-pipe turned, 

 and water poured into the other till it 

 runs out at the cock, which indicates 

 that the boiler and pipes are fully charged, 

 when the cock is again closed. As water 

 expands l-24th part of its bulk when 

 heated to 212°, it follows that neither 

 boiler nor pipes should, when cold, be 

 filled to their utmost extent ; and should 

 they by accident be so charged, the utility 

 of an open expansion-tube is obvious. It 

 is seldom, however, that the water attains 

 a temperature of 212°, and hence the ex- 

 pansion is much less, perhaps not often 

 more than l-30th part of the volume, ex- 

 clusive of the increased capacity afforded 

 it by the expansion of the boiler and 

 pipes. The usual mode of attaching sup- 

 ply-cisterns is to place them in any con- 

 venient place near to and above the level 

 of the boiler, and to connect them by a 

 small pipe, say of half-inch bore, to al- 

 most any part of the apparatus; but if to 

 the return-pipe, so much the better, as 

 shown in fig. 351. Hood has suggested 



Fig. 351. 



a still better plan, namely, " to bend the 

 pipe attached to the cistern into the form 

 shown by x y, which is a preventative to 

 the escape of any heat or vapour at that 

 part, as the legs of the inverted siphon x 

 generally remain quite cold." 



The cause of circulation is thus de- 

 scribed by Ainger in "The Gardeners* 

 Chronicle : " — " The basis of the whole 

 system is this — that whenever a vessel of 

 any form, containing an elastic or non- 

 elastic fluid, is partially heated, motion 

 will immediately take place in the fluid ; 



Fig. 352. 



and so far from its being difficult to pro- 

 duce this motion, the only difficulty is to 

 prevent the movement. It occurs under 

 the slightest difference of temperature ; 

 and the result is, that in nature no fluid 

 matter is quiet. Warm water in an ex- 

 posed vessel is in constant motion down 

 the cooling sides, and up the protected 

 centre ; while liquids cooling and evapo- 

 rating, under the microscope, are seen to 

 describe the most extraordinary evolu- 

 tions. In short, the whole fluid world, 

 whether heating or cooling, maintains a 

 perpetual motion, which requires a few 

 simple considerations to direct to our own 

 purposes. In illustration of 

 the simplest form of circula- 

 tion, let fig. 352 represent 

 a section of any enclosed space 

 containing air, the opposite 

 sides of which have tempera- 

 tures differing in any sensible 

 degree, there will infallibly 

 occur in the air a continu- 

 ous movement up the warmer 

 and down the cooler sides, 

 as shown by the arrows ; 

 while, in the centre of the 

 space, the conflict of the as- 

 cending and descending currents will 

 create numerous little whirlwinds, which 

 will, to a certain extent, in- 

 Fig. 353. terfere with and check the 

 main currents. For this 

 reason, it is found that a 

 diaphragm, fig. 353, to se- 

 parate the two currents, pro- 

 motes the freedom and in- 

 creases the rapidity of the 

 circulation, by confining the 

 two streams within their pro- 

 per limits. 



"By this diaphragm, the 

 vessel has become converted 

 into an ascending and de- 

 scending pipe; the more rapid circula- 

 tion in which, spite of the increased sur- 

 face exposed to friction with the circulat- 

 ing matter, leads to the inquiry, What is 

 the nature and amount of that friction of 

 fluids in pipes, about which so much is 

 said, and so little generally understood % 

 In the first place, it appears to be for- 

 gotten that the friction of the fluid against 

 the pipe cannot be greater than that of 

 the fluid against itself, because, if it were 

 so, the moving fluid has only to leave an 



