28 



HOT WATER APPARATUS. 



the nearer to the surface the more motion will be obtained, and with one 

 pipe there must be a double current in it." This was the principle con- 

 tended "for by the late Mr. Bacon, who mistook the double cmrent, 

 that is, the hot water flowing along the top part of the pipe and the 

 colder retui'ning to the boiler near its lower side, and calculated that 

 water as a conductor of heat, by one particle giving off its heat to the 

 next, it would in time heat the whole column contained in the pipes, which 

 no doubt it would, but not with rapidity. This doctrine is contrary to 

 the rules laid down by the late Count Romford, who considered water as 

 a nonconductor ; but the experiments of Dr. Thompson, Mr. Nicholson 

 and others have proved to the contrary'. 



" 9th. 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 

 and angle in the pipes." 



The thirteenth paragraph of this excellent paper thus proceeds : 

 13th. Having considered the circumstance necessary to the motion of 

 the fluid in pipes, the next inquiry must be the quantity of heat a Hquid 

 can convey in a given time, and the quantity of surface required to com- 

 municate 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 the same quantity again in coohng to its former 

 temperature, less nor more it cannot afford. It is equally true, that with 

 the same temperatures, equal and like smfaces give off equal quantities of 

 heat to air, and consequently, the quantities of heat exchanged under 

 given circumstances, are measurable quantities, and this subtile element is 

 brought within the domain of science." 



The conclusions arrived at by ^Ir. Tredgold on this subject are, that for 

 the generality of hot houses, double the number of feet contained in the 

 area of the surface of glass will be equal to the number of cubit feet of 

 air, which that surface (that is, the sm-face of pipes,) should heat per 

 minute when in full action ; but the surface of the apparatus will give off 

 heat of different degrees of temperatiure according to the materials they 

 are formed of and the different liquids they are charged with. Thus 

 thirty two and a half feet of pipe charged with linseed oil, will heat a 

 space equal to that heated by one hundred and six feet when brine is 

 employed, or one hundred and sixteen feet when clear water is used. The 

 advantage, therefore, of using a fluid capable of bearing a high temperature 

 vvithout boning is very considerable in lessening the quantity of surface 

 of pipe required, but oil is of too inflamable a nature to be used for general 

 purposes with safety. And in regard to the materials the pipes are 



