By Mr. Thomas Tredgold. 



577 



length, and inversely as the diameter of the pipes ; it is also 

 increased by every bend and angle in the pipes. 



13. Having considered the circumstances necessary to the 

 motion of the fluid in pipes, the next inquiry must be the 

 quantity of heat a liquid can convey in a given time ; and the 

 quantity of surface required to communicate it to the air of 

 the house. 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 cool- 

 ing to its former temperature ; less nor 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 ele- 

 ment is brought within the domain of science.* 



14. I have shown how to estimate the quantity of heat re- 

 quired for a Forcing-house, in my book on Warming and 

 Ventilating (Art. 70 and 71) and, have since had to assign 

 the proportions for houses of the largest size, and the most 

 different from the ordinary forms of any in this country, 

 which having stood the test of experience, and being far be- 

 yond the bounds of common practice, afford a proof of the 

 benefit of studying first principles in new and untried cases ; 

 but, in general, for Hot-houses, twice the number of feet con- 

 tained 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; 



* Sir Isaac Newton first established the laws of heating and cooling, in the 

 rhil. Trans, for 1701. 



vol. vii. 4 F 



