APPLICATION OP HOT WATER IN HEATING HOT-HOUSES. 
103 
7thly. In a close boiler a pipe may, at any distance from the boiler, rise to any 
height and descend again ; but it must neither rise twice, nor dip after leaving the 
boiler ; when it is necessary to raise it, there should be an open pipe inserted at 
each extremity of the height of the rise ; advantage has been taken of this circum- 
stance to avoid doorways. (Fig. 6.) 
Fig. 6. 
8thly. A certain quantity of motion would be obtained by a single horizontal 
pipe between any joints except the bottoms of vessels ; but the nearer to the 
surface the more motion will be obtained ; and with one pipe there must be a 
double current in it. 
9thly. The retarding effect of friction is directly as the length, and inversely as 
the diameter of the pipes ; it is also increased by every bend angle in the pipes. 
It is a fact, not so generally known as it ought to be, that if we communicate 
a certain quantity of heat to a liquid, it will give out exactly the same quantity 
again in cooling to its former temperature ; less or more it cannot afford. It is 
equally true, that, with the same temperatures, equal and like surfaces give off 
equal quantities of heat to air, &c., and consequently, the quantities of heat 
exchanged under given circumstances are measurable quantities, and this subtile 
element is brought within the domain of science *. 
In general, for hothouses, twice the number of feet contained in the area of 
the surface of glass will be equal to the number of cubic feet of air which that 
surface should heat per minute, when in full action. 
Now the heat given off by the surfaces of the apparatus depends on the kind 
of materials they consist of. When bright tinned iron, earthenware, &c, are 
employed for pipes, much more surface is necessary. 
If the cubic feet of air to be heated per minute be multiplied by the number 
of degrees it is to be warmed, and the result be divided by twice the difference 
between the temperature of the house and that of the surface of the pipes, the 
result will be the feet of surface of iron pipe, &c, required. 
It is known from experience, that the heat which raises the temperature of one 
cubic foot of water one degree, will heat 2850 cubic feet of air one degree. 
* Sir Isaac Newton first established the laws of heating and cooling, in the Phil. Trans, for 1701. 
