CAUSE OF CIRCULATION OF HOT WATER. 



269 



indefinitely thin film of its own substance 

 against the surface of the pipe, and the 

 resulting friction would then be fluid 

 against fluid." After proving, by examples 

 and arguments of sufficient force, that the 

 friction of water against water is greater 

 than against foreign bodies, he proceeds 

 to say — "The inference deducible from 

 these facts in relation to our present pur- 

 pose is this, that in every hot-water cir- 

 culation, there should be the least possi- 

 ble liability to produce intestinal currents 

 or eddies, that the water, as much as may 

 be, move in a mass. With this view, 

 sharp and numerous bends in the pipes 

 should be avoided ; and, above all, we 

 should guard against strictures, which 

 wiredraw the water, and create a quantity 

 of inter-friction, infinitely more prejudi- 

 cial than that against the sides of the 

 pipes. With well-arranged pipes, then, 

 the friction is doubtless very small, but 

 the inertia of the water is an obstacle of 

 considerable amount, which, like the fric- 

 tion, will increase with every change of di- 

 rection, with bends, and with strictures." 



It is difficult to calculate the amount 

 of resistance caused by friction and iner- 

 tia, either in rapid or sluggish circulation; 

 for both are affected by a variety of cir- 

 cumstances, neither easily understood nor 

 computed. " We are obliged, therefore," 

 says Ainger, "to estimate the quantity of 

 resistance, by knowing that it is certainly 

 below that of the force which overcomes 

 it, and which is more easily estimated. 

 In any system of pipes, however compli- 

 cated or numerous, if the sum of the pro- 

 ducts of the specific gravities of the 

 ascending columns, multiplied by their 

 several lengths, measured vertically, be 

 less than the corresponding sum of the 

 descending columns, motion will result, 

 which will be more rapid as this differ- 

 ence is greater." Ainger's theory of cir- 

 culation will be readily understood by a 



Fig. 354. Fig. 355. 



Fig. 356. 



I I 



! 1 



containing water, which simply illustrate 

 the principle of the movement. In all of 

 them, the left-hand side, or that having 

 an ascending arrow, is supposed, from 

 some cause, to be the warmest. In fig. 

 354, the ascending and descending cur- 

 rents would interfere with each other, 

 as before mentioned, creating whirls 

 and eddies which would check each 

 other's movement. In fig. 355, the dia- 

 phragm, by preventing this intermixture, 

 and by preventing also the exchange of 

 temperature otherwise than by circula- 

 tion, would greatly increase its velocity. 

 And in fig. 356, the velocity will be still 

 farther increased by the horizontal dis- 

 tance between the ascending and descend- 

 ing columns, which would cause them to 

 exhibit greater differences of specific 

 gravity. 



" Here, then, we see gradually produced 

 the three elements of power. 1. Freedom 

 from intestinal movements. 2. Substi- 

 tuting the friction of water against some 

 other substance, for that of water against 

 water. 3. Difference of specific gravity, 

 by causing the water to become much 

 cooler in the descending than in the 

 ascending chamber. We have now only 

 to imagine this last chamber to be closed, 

 fig. 357, and unequally divided, forming 



Fig. 357. 



bcl 



glance at the annexed diagram, figs. 354, 

 355, and 356, "representing three vessels 



what may be called a boiler at b, with a 

 cistern at e, and we have a hot-water ap- 

 paratus of the simplest form." 



The clear and lucid explanations given 

 by Mr Ainger in his various papers in 

 " The Gardeners' Chronicle," from which 

 the above, with the diagrams, is taken, 

 show a thorough practical, as well as the- 

 oretical, acquaintance with this subject, 

 and a general agreement in principle with 

 the views of Hood and others. 



On the operation of circulation, Ber- 

 nan observes, ("Hist, of Heating," &c.) — 

 " In all the apparatus which have been 

 described, the circulation of the water is 



