266 HEATING AS APPLIED IN HORTICULTURE. 



4-inch pipes should be placed centre from 

 centre to effect a perfect circulation under 

 ordinary circumstances — that is, where 

 the pipes are placed so as not to fall 

 below the horizontal level — then, in such 

 cases, the ascending pipe should in gene- 

 ral be just as much greater than the above 

 dimension as the depth which the circu- 

 lating pipe is required to be below the 

 horizontal level, taking into account the 

 length and diameter of the pipe through 

 which the water passes, as on this depends 

 the motive power, on account of the 

 difference of temperature between the as- 

 cending and descending columns of water. 

 If the pipe be of considerable length, a 

 less extent of vertical pipe will be re- 

 quired; but if the pipe be shorter, a 

 greater height must be allowed, taking 

 into consideration, at the same time, the 

 temperature that surrounds the pipes — 

 for on this depends the amount of heat 

 given out, which also affects the tempera- 

 ture of the descending pipe. Thus, sup- 

 pose the depth of the dip, shown by the 

 dotted line a b, as in the diagram an- 

 nexed, fig. 347, "to be 24 inches, then 



Fig. 347. 



(r 



e f 



I i 



\ 



g I A \ 







i) 





• c b 



the distance y z ought to be 40 inches, 

 if the pipes be 4 inches diameter — that 

 is, 36 inches from centre to centre, or 

 40 inches from the top of the pipe y to 

 the bottom of the pipe z; and, with these 

 dimensions, as good a circulation will be 

 obtained as when the distance between 

 the top and bottom pipes is 16 inches 

 from centre to centre in the common 

 form of the apparatus. It will be ob- 

 served that by this arrangement the dis- 

 tance c d, from the under side of the flow- 

 pipe to the upper side of the return-pipe, 

 is just 12 inches, which is the same height 

 that was stated (vide Boilers and Pipes) 

 to be necessary to secure a good circula- 

 tion on the ordinary plan without a ver- 

 tical dip. The reason why this height is 

 sufficient in the present case, notwith- 

 standing the increased friction of the 



angles, is because there must always be a 

 greater difference between the tempera- 

 ture of e and / than between g and h, 

 or between i and Jc, or even more than 

 between these together ; therefore the 

 tendency to direct motion is greater than 

 towards retrograde motion in proportion 

 to this difference, and is sufficient to over- 

 come the increased friction caused by the 

 vertical declination, while the additional 

 height of 12 inches beyond the height of 

 the dip, possessed by the descending pipe /, 

 is sufficient to produce circulation of the 

 water. If g and h, and also i and k, were 

 very wide apart — say 40 or 50 feet — instead 

 of being, as usual, only about 3 or 4 feet, the 

 balance of effect, though still in favour of 

 direct motion, would not be so great as 

 in the last supposed case, because there 

 would be a greater difference in tempera- 

 ture between g and h, (that is, h would be 

 heavier than g in a greater degree,) which 

 would give a greater tendency to retro- 

 grade motion. In extreme cases, there- 

 fore, it will be advisable to make the 

 ascending pipe somewhat higher in pro- 

 portion to the dip than is here stated, 

 probably when there are several such 

 alterations required in the level of the 

 pipes ; and, in all cases, as has been before 

 observed, the higher the ascending pipe 

 is made, the more rapid will be the cir- 

 culation." — Hood. 



On the subject of carrying hot water to 

 heights above the level of the boiler, as 

 well as depressing it to a lower level, Mr 

 Ainger gives the following statements in 

 "Gardeners' Chronicle." "If," says he, 

 " it be required to discontinue the line of 

 pipes in order to pass a doorway or other 

 obstacle, it is best done by an ascent, as 



Fig. 348. 



shown by the dotted lines in, in fig. 348, 

 which, if the pipes be kept close at the 



