HORTICULTURAL SOCIETV OF LONDON. 



147 



cumstances very rapidly, five or six times as much fuel as is 

 really necessary will be consumed by a gardener zealous of the 

 honour of his apparatus. 



In the opinion of the author, if a house is to be heated rapidly, 

 the pipes should be of the smallest diameter which is consistent 

 with a free circulation, but it must be borne in mind that such 

 pipes will Also cool with equal rapidity 3 and, if the heat is to be 

 maintained through the night, the furnace must be so constructed 

 as to contain a large quantity of fuel, and only to allow of a 

 very slow consumption, much after the manner of Dr. Arnott's 

 Stove. But there are .several objections to small pipes, one of 

 the most material of which is this, that the motion of water 

 within them being retarded by friction in a much greater degree 

 than in large pipes, they can never be brought to so high a mean 

 temperature. 



But it is very doubtful whether a rapid communication of heat 

 be really essential to the efliciency of a heating apparatus. In 

 hot-houses, where permanent heat is required, it is evidently un- 

 necessary j the only place where it may be desirable is in build- 

 ings where occasional heat only is employed. But if any one 

 will take the trouble to note hourly the variations of the ther- 

 mometer by night, in weather in which frost is so severe as to be 

 dangerous,, they will find that, instead of a sudden jump of 10° 

 or 20°, the thermometer begins to fall slowly an hour before 

 sunset, somewhat more rapidly afterwards, and continues falling 

 steadily till about 1 1 p.m. After that time it falls still more slowly 

 tills or 4 A.M., by which time it will have almost reached its mini- 

 mum. Its variation will be something like three or four degrees 

 per hour for the first four hours, after that about one degree per 

 hour for the next two or three, and then from J to J of a degree 

 till it has reached its minimum. Now it is evident that to meet 

 this variation, supposing the temperature of the house to range 

 exactly with outer air, an apparatus which occupies three or four 

 hours in reaching its maximum, would be much more accurately 

 adapted to the emergency than one which could be heated in an 

 hour. But in fact, except in iron-roofed houses, the temperature 

 within the house does not keep pace with that of outer air, but 

 falls much more slowly, owing to the specific heat contained in the 

 objects within the building, which is gradually transmitted by the 

 roof, so that in fact the necessity for rapid heating, even in green- 

 houses, is really less than at first sight appears. 



Mr. Rogers considers that the real desideratum is a furnace so 

 constructed that it shall contain fuel enough to supply the pipes / 

 with as much heat as they can radiate during the night, and 

 which may be depended upon for burning steadily and perfectly 

 whatever fuel is put into it. 



The author next proceeded to consider the best capacity for . 

 pipes employed in heating hot-houses and greenhouses. He 



