HEATING BY HOT WATER. 311 



it some coloured particles of matter, and tlien immersing the 

 tumbler in a vessel filled with hot water. It will at once he 

 seen, by the motion of the particles of coloured matter, that 

 at the sides of the tumbler there is an upward current of 

 heated, and in the centre a downward current of colder, water, 

 This goes on until the whole is of the same temperature. A 

 glass of warm water immersed in cold has the current reversed 

 in its course — upwards in the centre, and downwards at the 

 sides, where the water is being cooled. Here is the whole 

 secret of the motion and course of heated water ia the boUer 

 and pipes of a properly-adjusted heating apparatus. And 

 one would suppose that the simple understanding of this 

 would prevent any from making mistakes. Yet, strange to 

 say, some who undertake hothouse-heating are entirely igno- 

 rant of these simple and well-established facts. 



Wherever the heat generated by combustion in the furnace 

 acts most directly and powerfully, from that surface bound 

 upwards the particles of water, and to that spot, simulta- 

 neously, drop the colder particles of water, to be in. their 

 turn sent bounding on their errand of warmth. Anything 

 that attempts to contravene this law of gravitation will 

 be rebelled against by the elements concerned with un- 

 mistakable violence and persistency. Clearly, then, the 

 outlet for the water, thus lightened and charged with its 

 freight of heat, should be at the highest part of the boUer ; 

 and that by which the cold water is to run in and down, to 

 take its place, should be at the lowest point. Boiler inven- 

 tors and manufacturers recognise this important part of the 

 matter, and always place the flow-pipe at the highest, and 

 the return-pipe at the lowest, point of boilers. 



Great importance has been attached by many to the ne- 

 cessity, or at least the great desirability, of having the boiler 

 fixed at a very much lower level than the pipes ; and also 

 to the necessity of laying all the flow-pipes on the incline 

 the whole length of the house to be heated. The importance, 

 too of having the return-pipes on a considerable decline, has, 

 in my opinion, been very much over-estimated. It is en- 



