160 Wisconsin Academy of Sciences^ Arts, and Letters. 



pressure. The opening of the passage way again affords a tempo- 

 rary relief. Thus the boiler dilates and contracts to such, an ex- 

 tent that the movements ar'e sometimes visible to tbe eye, and they 

 have been compared to the breathing of some large animal. 



"With this slow and continuous change, there is no wonder that 

 boilers eventually "give out." If there is any mystery in the 

 case, it is that they last so long and serve so well as they d j. 



That steam and water in pipes not properly drained have great 

 percussive action, may be readily seen from the jumping and 

 snapping of the pipes under these conditions, and many serious 

 accidents have occured from pipes and fittings bursting, even loss 

 of life resulting in some cases. With these facts before us, great 

 care should be exercised, not to open the steam passages from the 

 boiler, too suddenly, on account of the danger arising from re- 

 lieving the pressure on the water. 



What effect might be caused by such lack of care, may be seen 

 in the following deduction. 



The heat required to raise one pound of water through one de- 

 gree of temperatui'e is termed a unit of heat, or its equivalent, 

 100 pounds of water through one-tenth of a degree, or one-tenth 

 of a pound through 100 degrees. This quantity of heat possesses 

 the same amount of power as would be required to raise 772 

 pounds, one foot, or one pound 772 feet. This is termed the 

 mechanical equivalent of heat. Now if the addition of one de- 

 gree of heat to one pound of water, be such an accession of force, 

 the addition of 100 degrees to 500 pounds of water is an equiva- 

 lent of a half million times that force. In practice, the combus- 

 tion of a pound of coal imparts to the water in a good boiler 

 about 10,000 units of heat, and evaporates eight or nine pounds 

 of water of usual temperature. "With all the losses and disad- 

 vantages considered, a pound of coal exerts about one-fourth of 

 a horse power per hour, fifteen horse power for a minute or 900 

 for one second. The heat absorbed by 5,000 pounds of water in 

 raising it through 100 degrees, is really twelve and a half horse 

 power for an hour, 750 for a minute or 45,000 horse power for a 

 second. The amount of heat absorbed by 5,000 pounds of water 

 in raising it through 100 degrees, is but a small portion of the 



