THE HEATING AND VENTILATING OF HOTHOUSES. 
323 
THE HEATING AND VENTILATING OF HOTHOUSES. 
By Mr. A. Donald Mackenzie. 
[Read December 4, 1900.] 
With the enormous increase in the prosperity and wealth of the country 
(luring the last fifty years, hothouses for the cultivation of fruits and 
flowers have increased in a full proportion. The maintaining of an 
equable temperature in such houses when the outside temperature varies 
sometimes as much as 20 to 30 degs. in twenty-four hours is not without 
difficulty. 
The means used to accomplish this is in nearly every case now hot 
water circulating m pipes ; the days of the old brick flues have gone. 
The theory of the circulation of hot water in pipes is very interesting, 
for we are presented with an apparent anomaly by the rapid ris8 of the 
Fig. 164.— Boilers, showing Flow and Return. 
water in the flow-pipe, apparently against the universal law that water 
flows to the lowest point — finds its level. But this is not the occasion 
for discussing this aspect of the question to any great extent. In passing, 
however, I may be allowed very briefly to draw attention to the cause of 
the circulation in a hot-water apparatus. 
Fig. 164 represents an ordinary apparatus with a saddle boiler to which 
is attached in the ordinary way a flow- and return-pipe ; the flow in all 
cases and in all classes of boilers must be from the highest available 
point of the boiler, and the return should rejoin the boiler as near the 
bottom as practicable. There is thus an endless pipe, the boiler being 
practically a part of the pipe enlarged and shaped for the application of 
heat. When such an apparatus is filled with water through the cistern 
and feed-pipe it is ready for use. When heat is applied to the part of 
the endless tube called the boiler what happens is this, the water 
expands — expands equally in all directions, downwards as well as up- 
wards ; but inasmuch as there is less resistance in the upward direction 
the whole expansion is diverted that way, the longer or rather higher 
column of water in the return-pipe resists the push of the expansion, 
K 
