HOT- WATER BOILERS AND PIPES. 



185 



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the difTerence between the temperature of the flowing 

 returning columns is 8 degrees, the difference in weight is 

 grains on each square inch of the section of the return- 

 supposing the height of the boiler A (Fig. 36, 

 B) to be 12 inches. This height, however, is 

 only taken as a convenient standard from 

 which to calculate ; for, probably, the height 

 may, in many instarices, be more thari this, 

 thoua^h it will seldom be less. 



Now, suppose that, instead of 12, 18 

 inches was the distance between the two 

 pipes, that is, between the top of the upper 

 and the centre of the lower pipe, and the 

 pipe 4 inches in diameter; if the difference of 

 temperature between the water in the boiler 

 and the return-pipe be 8 degrees, the pressure 

 on the return-pipe will be 153 grains, or 

 about one third part of an ounce; and this 

 will constitute the whole amount of motive 

 power of the apparatus, w^hatever be . the 

 length of pipe attached to it. If such an 

 apparatus have 100 yards of pipe 4 inches in 

 diameter, and the boiler contains, say, 30 gal- 

 lons of water, there will be in all 190 gallons, 

 or 1900 lbs. weight of water, kept in continual 

 motion by a force equal only to one third of 

 an ounce. This calculation of the motive 

 power will vary under different circumstances; 

 and, in all cases, the velocity of the circula- 

 tion will vary s'multaneously with it. 



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