580 Application of Hot V/ater in Heating Hot-houses. 



contain, is double the quantity cooled during the time of 

 making one circuit in the pipes, which is found by dividing 

 the quantity w, as found above, by the number of circuits, or 

 parts of a circuit made in a minute, and comparing the velo- 

 city with the length of the pipes. Whatever the quantity is 

 in excess above, this is to be considered a reserve of hot fluid 

 to afford heat after the fire is out ; and, the fire must be so 

 much earlier lighted as to heat this excess of water ; as it 

 must be hot before the surface can afford its effective supply 

 of heat. Hence, there is a considerable objection among 

 gardeners to large boilers, and large reservoirs. 



19. But the most important of the properties of the hot 

 water method, as first tried, consists in the power it has of 

 keeping up the temperature of the house for a long period 

 without attention from the attendant ; and, it is entirely owing 

 to the excess of fluid that it has this advantage over steam 

 heat ; and the exact knowledge which we now have of the 

 heat which water contains in proportion to its temperature, 

 enables us to calculate the time the cooling of the fluid will 

 maintain the heat of a house ; for if u be the number of de- 

 grees, the water is above the temperature of the house, and 

 w its quantity in cubic feet ; then since is the quantity to 

 supply the house one minute, 285 °^ M — the minutes the tem- 

 perature of the house will be sustained by the cooling of the 

 water longer than in a house heated by steam alone. It will 

 be obvious, the actual time of cooling will be more than twice 

 this time, and the heat afforded to the house will decrease, 

 but this is in some measure compensated for by the solid 

 parts of the house receiving an excess while the apparatus is 



