HEATING HOT-HOUSES BY HOT-WATER. 
129 
9thly. The retarding effeet 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 he, 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 un¬ 
der given circumstances are measurable quantities, and this subtile 
element is brought within the domain of science*. 
In general for Hot-houses, 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. 
19 
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 ne¬ 
cessary. 
If the cubic feet of air to be heated per minute he multiplied by 
the number of degrees it is to be warmed, and the result he divided 
by twice the difference between the temperature of the house, and 
that of the surface of the pipes, the result will he the feet of surface 
of iron pipe, &c. required. 
It is known from experience, that the heat which raises the tem¬ 
perature 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. 
