HEATING BY HOT WATER. 315 



the stream of hot water. Both have a hand in it, no doubt. 

 This influence of heat upon water can be very manifestly 

 shown by filling a tumbler with cold water, and mixing with 

 it some coloured particles of matter, and then immersing the 

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

 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 in the boiler 

 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 fur- 

 nace acts most directly and powerfully, from that surface 

 bound upwards the particles of water, and to that spot, 

 simultaneously, drop the colder particles of water, to be in 

 their turn sent bounding on their errand of warmth. Any- 

 thing that attempts to contravene this law of gravitation will 

 be rebelled against by the elements concerned with unmis- 

 takable 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 boiler ; and that 

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

 place, should be at the lowest point. Boiler inventors and 

 manufacturers recognise this important part of the matter, 

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

 turn-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 



