210 THE GARDENER. [May 



whether the rise in the pipes is made by a slow gradient, or by a verti- 

 tical rise, will not make material difference to the circulation ; but 

 every foot that they are thus elevated above the boiler gives greater 

 force to the circulation of the water." This sentence is a very import- 

 ant one bearing on the subject under discussion, and if its conclusions 

 are right, then all I have said on the matter is wrong, and therefore 

 worthless. Let us examine the first clause. It says — whether the rise 

 in the pipes is made by a slow gradient, or by a vertical rise, will not 

 make material difference to the circulation. Now, I maintain that, 

 when the highest point of the apparatus is reached by a slow gradient, 

 the result is a return-current in the flows. In an apparatus of a few 

 hundred feet of piping, this return-current is not so marked as in an 

 apparatus where the piping extends to thousands of feet ; but it occurs 

 in both. The water owes its expansion and relative lightness to heat 

 in the first instance, and as heat fails, the water contracts and becomes 

 relatively heavier; it therefore follows that the heated volumes of 

 water should reach the highest points of the apparatus before any 

 diminution of their temperature takes place ; and when this point is 

 reached by a slow gradient of, say 200 feet, I think Mr Inglis will ad- 

 mit that they cannot do so without becoming colder than they were at 

 the time of leaving the point on which the fire acts. And I think he 

 will also admit that every inch the water has to be raised after it com- 

 mences to cool is an extra tax on the imshing and pulling powers of 

 the colder and relatively heavier water in the return-pipes. The slow 

 gradient is therefore to be avoided as much as possible, as it leads to a 

 waste of force, and, consequently, retards circulation. 



We will now look into the last clause, which says that every foot the 

 flow-pipes are elevated above the boiler, gives greater force to the cir- 

 culation of the water. Mr Inglis is not singular in holding this opinion. 

 Hot-water engineers are very decided about elevation of the flows above 

 the boiler being essential to rapid circulation of the water in the ap- 

 paratus. They will sink a hole in the earth 20 or 30 feet deep — unless 

 prevented by some insurmountable obstruction — to obtain elevation for 

 the flows above the boiler, and after the flows reach the surface from so 

 low a level, they are again elevated to their farthest points from the boiler 

 in the various compartments to be heated, so that circulation may be 

 more rapid still. Thus it might be supposed that the rapidity of cir- 

 culation was proportionate to the elevation of the flows above the point 

 on which the fire acts. Let us see if this is really so, by supposing 5 

 feet to be the distance to which the flows are elevated above the boiler in 

 an apparatus of, say 3000 feet of piping, and that it takes an hour and 

 a-half from the time the fire is lighted until all the water in the appa- 

 ratus has passed through the boiler. Now, if the elevation had been 

 10 feet instead of 5 feet, would the water have made the circuit in less 

 time — say an hour and a quarter ; and if the elevation had been 20 feet, 

 would it have performed the journey in three quarters of an hour 1 If 



