72 On the Strength of Cylindrical Sieam Boilers. 
2 30 X 1000 
an inch thick? Here s= 35 =60,000. Hence we see that 
the metal must be capable of sustaining sixty thousand pounds to the 
inch bar, or in that proportion, for any other size. This formula en- 
ables us to determine whether among the metals of known tenacity 
any one can be found to fulfil the conditions under the thickness as- 
signed. ts 
12. On the basis of the foregoing formulas, the following table of 
diameters, thicknesses of iron, and strains to the inch of metal, in both 
directions, has been formed. It is obvious that the actual tenacity of 
the metal employed in a given case must be of the greatest impor- 
tance to the result. The extensive series of experiments recently 
undertaken by the Institute to determine this question, in reference 
to different kinds and varieties of boiler plate, and with regard to the 
various circumstances of its manufacture and application, will here- 
after furnish us with important data to aid in applying the formulas 
to each separate case. I shall for the present assume the tenacity 
of an inch square bar of rolled iron at 55000 Ibs. in the direction of 
the length of the sheet. Supposing the pressure generally employed 
in cylindrical high pressure boilers to be 150 Ibs. to the square inch, 
agreeably to the practice in this city, the table is calculated upon the 
principle that the boiler ought to have five times as great a strength 
as it is ordinarily required to exert. The calculation is upon a con- 
tinuous sheet of metal, without seams in any direction. The thick- 
nesses are given in ¢en-thousandths of an inch; but in practice the 
last figure may be omitted without material error. 
