i3 7 9-] HEATING BY HOT WATER. 371 



cause the water to become relatively lighter than it was before heat 

 was applied and expansion took place. Cold and contraction cause the 

 water to become relatively heavier than it was before cooling and con- 

 tracting took place. Now it requires both agents — heat and cold — to 

 maintain circulation; neither can do so independent of the other. It 

 requires both to bring about the immediate cause — not the primary 

 reason — of the water circulating through the apparatus. In the case 

 of the hot-water apparatus, we make heat and expansion the first pro- 

 moters of circulation, because we want to heat our plant-houses; but 

 on a hot summer day, if we wanted to cool them, we would make cold, 

 or rather the absence of heat and contraction, the first promoters of cir- 

 culation, and in place of using fuel under the lowest point of the appar- 

 atus, we would use ice of as extreme cold as we could get. By the by, 

 may I ask " C. M." how many degrees below zero does he fix " extreme 

 cold, as the absence of heat? " On the top of the highest point circu- 

 lation would take place just the same as if combustion was going on 

 below the boiler, which shows that heat and expansion have no more to 

 do with circulation than cold and contraction. I need scarcely point 

 out that " C. M." is mistaken in supposing that expansion "moves in 

 one direction only — that is, towards the highest point of elevation. " 

 If the force of expansion was towards the highest point only, it would 

 not be necessary to have the bottom of a steam-boiler as strong as its 

 top. Expansion, however, is from the centre, and acts with equal force 

 in all directions, therefore all parts of a steam-boiler are made, or ought 

 to be, equally strong. 



" C. M." says the relatively heavier water in the flow " must either 

 be forced or drawn uphill ; " and, " to prove that it is not drawn," he 

 points out that " there are generally air-pipes at the highest point of 

 elevation, and before the heavy column in the return would draw up 

 the column in the flow, the air would rush in and fill up its place." 

 This may furnish a reason for the incorrectness of the opinion that 

 circulation is principally due to the relatively heavier column; but I 

 think those who hold this opinion will not consider the reason con- 

 clusive that they are wrong; and if " C. M." has no other but this air- 

 pipe proof to offer in support of his statement that the relatively heavier 

 water in the flow-pipes " must be forced uphill " by the relatively lighter 

 water, then I submit he has failed in his effort to prove what he under- 

 took to prove. How could air rush into'an endless tube full of water, 

 and the feed-cistern higher than the highest point of the apparatus 1 

 In a properly-fitted-up hot-water apparatus, neither column pushes, 

 pulls, forces, or draws the other: both columns move contemporane- 

 ously; and when either column has to push, pull, force, or draw the 

 other, there is a defect in the fitting-up of the apparatus at some point. 



We now come to where " C. M." uses the word "absurdity." It is 

 not quite clear to me whether he intends absurdity to apply to his 

 statement or to the inference I drew therefrom. If to the former, I 



