1S74.] '^t)y [Cresson. 



age steam space above tlie crown-slieet is 10 inches, and tlie compression 

 of the 204 inches of steam into 10 inches would give a pressure of 20.2 

 atmospheres or 300 lbs. per square inch momentarily added to that of 90 

 lbs. already existing in the boiler. The cross-section of the strain above 

 the crown-sheet is 42 inches. Thickness of the iron j% of an inch, or total 

 section of iron to be broken f of an inch. Breaking weight from experi- 

 ment^54.400 lbs. per sq. in. 



54000 y, 5 \ Breaking strain of perfectly stayed=-:^°2~°=810 pounds 



) Br( 



I 1^ 



Then 



5^ t per sq. m 



Deducting 20 per cent, for rivets=648 lbs. per sq. in., or, according to 

 Fairbairn, deducting 44 per cent, for single riveted joints=454 lbs. for 

 bursting strains. 



Let us now suppose the crown-sheet and bars to have been heated to 

 a temperature of 660° Ft., and to be reduced to 485° Ft., the loss of tem- 

 perature— 175° Ft. Then 175 X 1,110 = 194,250 heat units, or --- 8 = 

 24,281 lbs. of water heated 1° Ft. 



Sensible heat of steam at 570 lbs. = ................ 485° 



" " " 90 lbs. = 3350 



Difference ...... 150° 



This added to the latent heat of steam at 570 lbs. = (777° + 150° = ) 

 927° as the units of heat necessary to convert water at 335° Ft. into steam 

 at 4850 Ft. 



Then 24,281 ~- 927 -= 26.19 lbs, of water converted. This amount of 

 water would cover the crown-sheet to a depth of 0.228 inches, and would 

 yield an amount of steam of atmospheric tension sufficient to occupy a 

 space equal to a prism having the crown-sheet for its base and 387 inches 

 in height. 



When compressed into a height of 10 inches this steam would sud- 

 denly add a pressure of 38 atmospheres or 570 lbs. per square inch to the 

 previous isressure of 90 lbs. per square inch, and would give, a total 

 pressure exceeding the 648 lbs. per square inch in the maximum of force 

 necessary to tear the boiler apart, as derived from experimental trials of 

 the strength of iron. 



I have endeavored by experiment to fix the temperature at which iron, 

 with such a surface as that from this boiler, will produce a spheroidal 

 state in water flowed upon it. When the samples of iron were floating 

 upon boiling mercury (662° Ft.) they failed to repel the water but con- 

 verted it rapidly into steam. 



The specific heat of iron, at high temperatures, averagiag about 0.122, 

 and its specific gravity 7.8, we can assume that an iron plate will raise the 

 temperature of a stratum' of water in contact with one surface, and of its 

 own thickness in depth, 1° Ft. for every degree that it loses in tempera- 

 ture, or that it will convert about j^^ou of that amount of water at the 

 boiling point into steam of high tension. 



