The Locomotive - Lighthouse Survival Blog

66 THE LOCOMOTIVE. [May,
much as we please, it is always a matter of opinion whether the strength obtained in
this way is sufficient to make u}} for that lost by cutting out the hole. We say it is a
matter of opinion, because, although there are rules for this re-enforcement, it does not
appear that these rules are based either on a mathematical analysis of the strains, or on
the results of extensive experiments. The mathematical questions involved in this
problem are of such a character that it is doubtful if many men are competent to handle
them; and those who are or have been competent seem to have overlooked the problem
f
Fig. 3. — Illustrating the Tendency to Deform the Dome.
or avoided it. So far as experiments are concerned, we should not know where to look
for satisfactory data of this kind, on which to base a rule. Such rules as there are seem
to be empirical, and must be regarded merely as an expression of what past experience
in the manufacture and use of boilers has indicated to be sufficient, supplemented, per-
haps, by a few general mechanical considerations of the most elementary nature. This
being the fact, it must be evident that the best thing to do, if domes are to be used at
all, is to so proportion the opening in the shell that there can be no reasonalJe doubt
about the strength of the structure; we must err on the safe side. Now certainly it is
not erring on the safe side to cut the opening in the shell the full size of the dome; for
whatever doubt there may be about the value of that part of the shell that lies within
the dome, it has never been maintained, so far as we know, that the shell is stronger
when this part is entirely removed than when it is left. The girthwise strain on a
boiler is well known to be double the lengthwise strain; so the tendency of the strains
in the shell is to distort the dome in such a way as to increase the diameter in a direction
perpendicular to the length of the boiler. (Fig. 2 illustrates this action. The opening