CULTIVATION OF THE GR4PB UNDER GLASS. 147 



But tliis is not all the "difference. The heat of the iron pipe must 

 also be taken into consideration, which, if calculated at four 

 inches in diameter and one-fourth of an inch in thickness, will, 

 in the case of hot water, contain 4.68 times as much heat as the 

 one filled with steam ; so that in fact if the pipe, when filled 

 with steam, cools down to 60° in one hour, it will require four 

 hours and a half to cool the iron to the same temperature when 

 filled with water. Ifor yet is this all. As soon as the water in 

 the hoUer falls below 212°, all circulation of steam ceases; but 

 in the case of water, the circulation is \ept up untU the water in 

 the boiler falls to the same temperature as that in the pipes. 

 Hence the temperature of the house is kept from falling below 

 60°, not only untU all the water in the pipes, and the pipes 

 themselves, have fallen to this point, but until all the water in 

 the boiler, and the boiler itself, has reached the same tempera- 

 ture. From these observations it will be seen that a house heated 

 with hot water will maintain its temperature six times as long as 

 one heated by steam. 



Again, in order to heat a building by steam, the pipes must 

 be above the temperature of 212°, and as we advance above this 

 point we soon reach such a degree of heat as we have already 

 described as being prejudicial to that purity of atmosphere so 

 essential to animal and vegetable life and health 



BOILERS AND PIPES. 



The efficiency of a boiler depends upon the quantity of sur- 

 face exposed to the fire, and that should be in proportion to the 

 amount of water contained in the boiler and pipes A bcUer 

 which has a surface of seven square feet exposed to the fire 

 will heat four hundred feet of four-inch pipe sufficiently for 

 practical purposes. It is better that the boiler should have a 

 capacity above the proportion of the pipes than below, for though 

 the circulation will be slower, the temperature can be maintained 

 at the desired point with a less consumption of fuel. The best 

 materia] for these boilers is cast iron ; they last longer than those 



