Jl. B. Quinby on high and lotv pressure'Boilers. 315 



it will appear from the above results, that the low pressure 

 engine is more than three times as safe as one of the high 

 pressure kind ; or, that the safety of the low pressure boiler 

 is to that of the high pressure boiler in the proportion of 16^ 

 to 5. 



But it can be shown that either engine can be made en- 

 tirely safe ; and that one kind is not, wt fact, any more liable 

 to explode than the other. 



To prove this we have, (by mechanics,) the thickness of a 



low pressure boiler 90 inches in diameter, capable of resist- 



-* , „ ■ u 45X10 ^,. . , 



me 10 lbs. per square mch, = =.015 m. ; and, 



^ * ^ 30,000 



that of a high pressure boiler 30 inches in diameter, capable 



15X1 50 



of resisting 150 lbs. per square inch = =.075 inches. 



^ r n 30,000 



And, now, if we multiply the first of these results by 10, 

 we shall have*. 15 in. for the thickness of a low pressure 

 boiler capable of resisting 10 times the usual working pres- 

 sure ; which is 90 lbs. above the usual working pressure. 



And the thickness of a high pressure boiler capable of 

 resisting 10 times the usual working pressure is 



^15 X150X 10^^75 j„ and, lastly, the thickness of a 



30,000 

 high pressure boiler capable of resisting 90 lbs. per square 



. , , ,, II- • 15X150+90 

 mch above the usual working pressure is = ' =; 



"" 30,000 



.12 in. 



Hence, as it is fully practicable to make a boiler of a thick- 

 ness equal to any of the foregoing results, it is plain that one 

 kind of engine may be made just as safe as the other. 



I shall merely add, that as the tenacity of metals is dimin- 

 ished by an increase of temperature, the tenacity in the case 

 of the high pressure boiler should have been taken some less 

 than in the case of the low pressure boiler.* 



* As no experiments have ever, to my knowledge, been made for de- 

 termining- the decrease of the tenacity of metals corresponding- with a 

 given increase of temperature, and as this, in my estimation, is a sub- 

 ject of considerable philosophical interest, I will respectfully sug-g-est to 

 the gentlemen who compose the present committee, that they have 

 such experiments instituted, as may, if possible, detect the law, by which 

 the tenacity decreases, during a given increase of temperature. 



