526 THE GARDENER. [Nov. 



place was fitted up on a wrong principle. We would be looking at an 

 apparatus in which the colder water was floating or forcing the hotter 

 water up out of the boiler into the flow-pipe j but once the hotter gets 

 into the flow-pipe, a change takes place, and the hotter forces the colder 

 uphill. Could any tiling be more condemnatory of the practice of 

 having the piping fixed with a continuous ascent than that, when so 

 fixed, the process of circulation takes place as here described 1 If it is 

 right both in theory and practice that the colder water should force 

 the hotter up out of the boiler, it is right that the colder should force 

 the hotter uphill at all points of the apparatus. The law is, that when 

 two fluids of different densities are in immediate contact with one 

 another, that of greater density descends and takes the lowest place, 

 and that of lesser density ascends and takes the highest. And there- 

 fore, if the hotter water lifts the colder at any point of the apparatus, it 

 is a proof that the method on which the latter is fitted up tends more 

 to hinder than to facilitate the process of circulation. 



Mr Makenzie tells us that " atmospheric air is lighter than water 

 bulk for bulk." This fact I think is pretty generally understood by 

 most people. But atmospheric air cannot force water hulk for bulk 

 uphill. It takes a column of atmospheric air somewhere about forty- 

 five miles high to balance a column of water somewhere about thirty- 

 five feet high. And a column of atmospheric air of the above height 

 could not force the water uphill one inch unless the pressure of the 

 atmosphere was removed by a mechanical or other contrivance from 

 the upper end of the tube in which the water is wanted to rise. 



The " suction-pump " theory of circulation will not work alike at all 

 points of the apparatus, nor will the", " strata" theory. We therefore 

 conclude that neither is the right theory. The latter works alike at 

 all points, and " does not take up a fragment of the subject and for- 

 get all the rest." 



I agree with Mr Henry J. Pearson that " glass houses are, or should 

 be, arranged so that plants may be grown successfully, with the greatest 

 economy of labour possible, not to put hot-water apparatus in ; and 

 a system which will not admit of doors and paths being placed where 

 required is at once condemned, even if the water does circulate the best 

 in that form." But if Mr Pearson thinks that the method of erecting 

 the hot-water apparatus recommended by me would be prejudicial to 

 the growth of plants, or interfere with the economical working of the 

 houses to which it was applied, he is mistaken in his conclusions. The 

 same facilities for having "doors and paths where required" are offered 

 by the method I advocate as are offered by the method in general prac- 

 tice. And by adopting the former we get over the necessity for those "un- 

 mitigated nuisances," as Mr Makenzie truly terms them — deep stoke- 

 holes, and for dipping and rising again of the pipes. When " dips " 

 occur, they render the circulation uncertain and extremely difficult. 

 The annexed figure will show how to heat a lean-to range of any 



