HOT-WATER BOILERS AND PIPES. . 187 



There aru two ways by which the motive power may be 

 increased. One, to allow the water to cool a greater number of 

 degrees between the time of its leaving the boiler and the period 

 of its return through the descending pipe. The other, by 

 increasing the vertical height of the ascending and descending 

 columns. The efTects produced by these two methods are pre- 

 cisely similar ; for, by doubling the difference of temperature 

 between the flow and return pipes, the same increase of power 

 is obtained as by increasing the vertical height. 



There are two methods of increasing the difference of temper- 

 ature between the flowing and returning pipes. First, by 

 increasing the quantity of the pipe, so as to allow the water to 

 flow a greater distance before it returns to the boiler. Secondly, 

 by diminishing the diameter of the pipe, so as to expose more 

 surface in proportion to the quantity of water contained in it, 

 and by this means to make it part with more heat in a given 

 time. 



The first of these methods, although the most practical, is 

 ncessarily limited, in some instances, to the length of the build- 

 ing to be heated, to which the length of pipe must be adjusted, 

 in order to obtain the required temperature ; and, as to the 

 second, we have already enumerated many objections against 

 the use of sm.all pipes. Where the motive power, therefore, is 

 not of sufficient strength, the increase of the height of the col- 

 umn ascending from the boiler must be depended on for an 

 additional motive power. 



In all cases, the rapidity of circulation is proportional to the 

 motive power, and, in fact, it is the index and measure of its 

 amount. For, if, while the resistance remains uniform, the 

 motive power be increased in any manner, or in any degree, 

 the rapidity of circulation will increase in a relative proportion. 



Now, the motive power may be augmented, as we have seen, 

 either by increasing the vertical height of the pipe, by reducing 

 its diameter, or by increasing its length. If, by any of these 

 means, the circulation be doubled in velocity, then, as the water 

 will pass through the same length of pipe it did before, in one 

 half the time, it will only lose half as much heat as in the for- 

 mer case, because the rate of cooling is not proportional to the 



