not require staying, while the upper tube-plate forming the top of the square internal flue would be strutted asunder and prevented from collapsing by the tubes themselves, some of which should be screwed into the plates or formed with internal nuts, to make them more efficient in this respect. Such a boiler would have various advantages over ordinary locomotive boilers, and might be made of any power that was desired without any limitation being imposed by the width of the gauge of the railway. Such boilers might also be used for steam-vessels by merely increasing the area of the fire-grate.

STRENGTH OF BOILERS.

The proportions which a boiler should possess in order to have a safe amount of strength will be determined partly by the pressure of the steam within the boiler, and partly by the dimensions and configuration of the boiler itself. The best proportions of the riveted joints of the plates of which boilers are made are as follows:

If the strength of the plate iron be taken at 100, then it has been found experimentally that the strength of a single-riveted joint will be represented by the number 56, and a double riveted joint by the number 70. According to the experiments of Messrs. Napier and Sons, the average tensile strength of rolled bars of Yorkshire iron was found to 61,505 lbs. per square inch of section,

STRAINS AND STRENGTHS.

321

and the average strength of bars made by nine different makers (and purchased promiscuously in the market) was found to be 59,276 lbs. per square inch of section. The tensile strength of cast steel bars intended for rivets was found to be 106,950 lbs per square inch of section, of homogeneous iron 90,647 lbs., of forged bars of puddled steel 71,486 lbs. and of rolled bars of puddled steel 70,166 lbs. per square inch of section. The strength of Yorkshire plates Messrs. Napier found to be-lengthwise 55,433 lbs., crosswise 50,462 lbs., and the mean was 52,947 lbs. per square inch of section. The tensile strength of ordinary best and best-best boiler plates, as manufactured by ten different makers, was found to be-lengthwise 50,242 lbs., crosswise 45,986 lbs., and the mean was 48,114 lbs. per square inch of section. Plates of puddled steel varied from 85,000 lbs. to 101,000 lbs. per square inch of section, and homogeneous iron was found to have a tensile strength of about 96,000 lbs. per square inch of section.

Experiments have been made to determine the strength of bolts employed to stay the flat surfaces of boilers together; and it has been found that an iron bolt 4ths of an inch diameter, like the staybolt of a locomotive, screwed into a copper plate ŝths of an inch thick, and not riveted, bore a strain of 18,260 lbs. before it was stripped and drawn out. When the end of the bolt was riveted over it bore 24,140 lbs. before giving way, when the head of the rivet was torn off, and the bolt was stripped and drawn through the plate. When the bolt was screwed into an iron plate ŝths of an inch thick, and the head riveted as before, it bore a load of 28,760 lbs. before giving way, when the stay was torn through the middle. When the staybolt was of copper screwed into copper plate and riveted, it broke with a load of 16,265 lbs., after having first been elongated by the strain onesixth of its length. Locomotive fire-boxes are usually stayed with 4-inch bolts of iron or copper pitched 4 inches asunder, and tapped into the metal of the outer and inner fire-boxes, and the stays are generally screwed from end to end. These stays give a considerable excess of strength over the shell, but it is necessary to provide for the risk of a bad bolt.

With these data it is easy to tell what the scantlings of a boiler should be to withstand any given pressure. If we take the strength of a single-riveted joint at 34,000 lbs. per square inch, then in a cylindrical boiler the bursting strength in pounds will be measured by the diameter of the boiler in inches multiplied by twice the thickness of the plate in inches, and by the pressure of the steam per square inch in pounds; and this product will be 34,000 lbs. Thus in a cylindrical boiler 3 feet or 36 inches diameter and half an inch thick, if we suppose a length of one inch to be cut off the cylinder we shall have a hoop an inch thick and 1 inch long. If we suppose one-half of the hoop to be held fast while the steam endeavours to burst off the other half, the separation will be resisted by two pieces of plate iron 1 inch long and an inch thick; or, in other words, the resisting area of metal will be one square inch, to tear which asunder requires 34,000 lbs. The separating force being the diameter of the boiler in inches multiplied by the pressure of the steam on each square inch, and this being equal to 34,000 lbs., it follows that if we divide the total separating force in pounds by the diameter in inches, we shall obtain the pressure of the steam on each square inch that would just burst the boiler. Now 34,000 divided by 36 (which is the diameter of the boiler in inches) gives 944-4 lbs. as the pressure of the steam on each square inch that would burst the boiler. A certain proportion of the bursting pressure will be the safe working pressure, and Mr. Fairbairn considers that one sixth of the bursting pressure will be a safe working pressure; but in my opinion the working pressure should not be greater than between one-seventh and one-eighth of the bursting pressure. The rule which I gave in my 'Catechism of the Steam Engine,' for determining the proper thickness of a single-riveted boiler, proceeds on the supposition that the working pressure should be of the bursting pressure. That rule is as follows:

TO FIND THE PROPER THICKNESS OF THE PLATES OF A SINGLE

RIVETED CYLINDRICAL BOILER.

RULE.-Multiply the internal diameter of the boiler in inches by the pressure of the steam in lbs. pcr square inch above the

Here 42 inches (which is the diameter) multiplied 80 = 3360, and this divided by 8900 = 377, or a little overan inch. The decimal 375 is of an inch.

Example 2.-What is the proper thickness of a single-riv cylindrical boiler 3 feet diameter, intended to carry a pres of 100 lbs. on the square inch?

As the double-riveted joint is stronger than the single-riv in the proportion of 70 to 56, it follows that 56 square inche sectional area in a double-riveted boiler will be as strong as square inches in a single-riveted. This relation is expressed the following rule :—

TO FIND THE PROPER THICKNESS OF THE PLATES OF A DOUB RIVETED CYLINDRICAL BOILER.

RULE. Multiply the internal diameter of the boiler in inche the pressure of the steam in pounds per square inch above atmosphere, and divide the product by the constant num 11140: the quotient will be the proper thickness of the bo in inches when the seams are double-riveted.

Example 1.-What is the proper thickness of the pl of a double-riveted cylindrical. boiler 42 inches diameter, intended to work with a pressure of 80 lbs. per square inch? Here 42 × 80=3360, and this divided by 11140:

='3 or about of an inch, which is the proper thickness of plates when the boiler is double-riveted.

6

Example 2.-What is the proper thickness of a double-riv cylindrical boiler 3 feet diameter, intended to carry a pressur 100 lbs. on the square inch?

Here 36 inches x 100 =

3600, which divided by 11140 = 322, or a little more than of an inch, which will be the proper thickness of the plates of the boiler when the seams are doubleriveted.

If Ꭲ ; the thickness of the plate in inches, D = the diameter of the cylinder or shell of the boiler in inches,, and P the pressure of the steam per square inch: Then

T=

DP

11140

=

is the formula for the thickness of single-riveted

is the formula for double-riveted boilers.

Moreover, in single-riveted boilers

These formulæ put into words are as follows:

TO FIND THE PROPER DIAMETER OF A SINGLE-RIVETED BOIIER

OF KNOWN THICKNESS OF PLATES AND KNOWN PRESSURE OF STEAM.

RULE.-Multiply the thickness in inches by the constant number

8900, and divide by the pressure of the steam in lbs. per square inch. The quotient is the proper diameter of the boiler in inches.

Example 1.-What is the proper diameter of a single-riveted cylindrical boiler composed of plates 377 inches thick, and intended to work with a pressure of 80 lbs. on the square inch?

Here 377 × 8900 = 3355·3, which divided by 80 = 41.94 inches, or 42 inches nearly, which is the proper diameter in inches.