HOT-WATER BOILERS AND PIPES. 191 



direct the application of those mechanical means to the exact 

 spot where they will be useful. We have frequently seen 

 mechanics, who, though well acquainted with the practical 

 details of the apparatus they were erecting, yet were perfectly 

 ignorant of the principles on which it works ; hence the success 

 of such an apparatus must be entirely a matter of chance. 

 Wherever alterations of the level occur, vents should be pr<2- 

 vided for the escape of air; and, as we have said, a small tap 

 (or cock) will be the most convenient method of outlet. 



In a complicated arrangement of hot-water apparatus, it is 

 sometimes so very difficult to detect the various causes of inter- 

 ference, and the impediments which arise are often so apparently 

 insignificant in their extent, that when ascertained they are 

 frequently neglected. Those, however, who bear in mind how 

 very small is the amount of motive power in any apparatus of 

 this description, will not consider as unimportant any impedi- 

 ment, however small, which they may detect ; moreover, they 

 will immediately see the propriety of having the evil in ques- 

 tion put right. But, in the more complicated forms of the 

 apparatus, so many causes become operative in impeding the 

 circulation, that the real cause of impediment may elude the 

 detection of even an experienced practitioner. 



We will now proceed to give a description, in detail, of various 

 methods of heating, which come within the range of our own 

 experience, accompanying the descriptions with sketches, by 

 which their details will be more easily understood. 



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