1879.] HEATING BY HOT WATER. 183 



tion. My opinion at the time was, that the water in the return-pipes 

 did not move back to the boiler as fast as the heating power of the 

 boiler required ; and the consequence was that the water in the 

 boiler boils, or becomes so disturbed as to cause the water to fly out at 

 the air-pipe. Some will be ready to say that this blowing at the air- 

 pipes takes place just the same in many instances where the pipes 

 are high above the boiler. This I admit, but think it is waste of 

 coals to have this go on, and further, that when a boiler is fed by a 

 ball-cock in the supply-cistern, or carefully attended to, air-cocks are 

 far better than the usual air-pipes. 



Mr Hammond and most gardeners know that when two or more 

 houses on different levels have to be heated from the same boiler, the 

 most heat always goes to the highest house, if there is no check put 

 on the circulation by means of valves. If two houses are fitted with 

 the same quantity of pipes, but the one higher than the other, and if 

 the one that is highest maintains the highest temperature, is it not 

 fair evidence of a quicker circulation of water in those pipes than in 

 those on the lower level?. The chief reason of this, I consider, is 

 because of the additional weight of water coming from the return-pipe 

 in the higher house over that coming from the lower house, and, in 

 consequence, its ijoiver to draw the hot inciter from the toiler in its 

 wake. The heating of the water and its consequent expansion and 

 lightness are the first motive power ; but I am disposed to attribute 

 the force of circulation more to the weight of cold water returning 

 to the boiler, and acting on the warm water in the boiler as a syphon 

 would do on a cistern of water. If you wish to empty a cistern of 

 water by means of a syphon, the length of time required to do so 

 will be much shortened by having the end of your syphon a good way 

 below the level of the cistern. And so, I think, it is with the circu- 

 lation in our boilers, — the greater the difference between the highest 

 point of the pipe and the bottom of your boiler, the quicker will the 

 circulation be. The water in a boiler at work being much lighter 

 than that in the return-pipes, and assisted by the agitation and ex- 

 pansion conveyed to it by the heat of the fire, circulation will take 

 place if the return-pipe is simply fixed immediately below the flow ; 

 but it would be slow. Mr Hammond, I think, makes too much of 

 the inclination of the water to form a return-current in the fiow- 

 pipes, and too little of the fact that the water in the return-pipes is 

 very much heavier — that this heavy column of water is putting, as it 

 were, a lighter one, and at the same time is pushing it from below. 

 Whether the rise in the pipes is made by a slow gradient or by 

 a vertical rise, I think, will not make material difference to the 

 circulation ; but every foot that they are thus elevated above the 







