CAUSE OF CIRCULATION OF HOT WATER. 



263 



frame being grooved to fit exactly; and 

 they are therefore nearly air-tight. They 

 also possess this advantage, that, should 

 explosion take place in consequence of 

 an accumulation of gases within the fur- 

 nace — and all furnaces and stoves upon 

 the principle of slow combustion are 

 liable to such accidents — instead of the 

 furnace being shaken by such explosion, 

 the doors would be lifted outwards, and 

 the evil avoided. 



§ 9. — CAUSE OF CIECULATION OF 

 HOT WATER. 



The inventors of heating with hot 

 water, and most of the writers on the 

 subject, appear to have held erroneous 

 notions on its principle of action. At- 

 kinson, Bacon, Barrow, Tredgold, and 

 others, believed that circulation was 

 caused by the water in the boiler expand- 

 ing when heat was applied to it ; and in 

 the appendix to Tredgold's work on heat- 

 ing, we find the following reasoning : — 

 " If the vessels A B, fig. 344, and pipes, 



Fig. 344. 



be filled with water, and heat be applied 

 to the vessel A, the effect of heat will 

 expand the water in the vessel A, and 

 the surface will, in consequence, rise to a 

 higher level a «, the former general level 

 surface being b b. The density of the 

 fluid in the vessel A will also decrease in 

 consequence of its expansion ; but as soon 

 as the column d c, above the centre of the 

 upper pipe, is of a greater weight than 

 the column / e, motion will commence 

 along the upper pipe from A to B ; and 

 the change this motion produces in the 

 equilibrium of the fluid will cause a 

 corresponding motion in the lower pipe 

 from B to A." This reasoning coincides 

 exactly with the opinions held by the 

 authorities above named. Mr Hood, 



however, dissents from these opinions ; 

 and after pronouncing them erroneous, 

 gives his theory in the following words : 

 — " Let us suppose heat to be applied to 

 the boiler a, fig. 345, a dilatation of the 



Fig. 345. 





-fl C < 



a 



f 







volume of the water takes place, and it 

 becomes lighter — the heated particles 

 rising upwards through the colder ones, 

 that sink to the bottom by their greater 

 specific gravity ; and they in their turn be- 

 come heated and expanded like the others. 

 This intestine motion continues until all 

 the particles become equally heated, and 

 have received as much heat as the fuel 

 can impart to them. But as soon as the 

 water in the boiler begins to acquire heat, 

 and to become lighter than that which is 

 in the opposite vessel b, the water in the 

 lower horizontal pipe d is pressed by a 

 greater weight at z than at y, and it 

 therefore moves towards a with a velocity 

 and force equal to the difference in pres- 

 sure at the two points y and z. The 

 water in the upper part of the vessel b 

 would now assume a lower level, were it 

 not that the pipe c furnishes a fresh sup- 

 ply of water from the boiler to replenish 

 the deficiency. By means of this unequal 

 pressure on the lower pipe, the water is 

 forced to circulate through the apparatus, 

 and it continues to do so as long as the 

 water in b is colder, and therefore heavier, 

 than that which is in the boiler ; and as 

 the water in the pipes is constantly 

 parting with its heat, both by radiation 

 and conduction, while that in the boiler 

 is as continually receiving additional heat 

 from the fire, an equality of temperature 

 never can occur ; or else, if it did, the 

 circulation would cease. 



" We see, then, that the cause of circu- 

 lation is the unequal pressure on the 

 lower pipe of the apparatus ; and that it 

 is not the result of an alteration which 

 takes place in the level of the water, as 

 has been erroneously supposed." 



From this it appears that circulation 

 is really owing to the water in the lower 



