ON THE STRENGTH OF PILLARS. 
409 
sions. Solid pillars may be made as small as may be desired, but this is not the 
case with hollow ones. These require great care on the part of the moulder, and 
are consequently expensive. The first hollow pillars used in these experiments were 
cast in a horizontal position, but it was found impracticable in this way to keep the 
core in the centre of the casting, and therefore several of the first pillars were de- 
fective, in having one side much thicker than the other, as well as in having many air 
bubbles in them. These causes tended to diminish the strength of the pillars; and 
will account for some results in Table VIII. being lower than they otherwise would 
have been. The circumstance of some of these pillars being much thicker on one 
side than the other, gave me an opportunity of observing what falling off in strength 
arose from that cause, which is one to which cast-iron pillars, as used in practice, 
must be very liable, though not so much so as in these small experimental castings. 
I have therefore recorded these cases with the others, conceiving that they would 
be interesting to practical men ; and it is gratifying to find that a matter, which would 
seem to destroy all confidence in a pillar, does not produce a great reduction in the 
strength. The cause seemed to be this : in almost every case, where pillars were 
much thicker, in the middle, on one side than the other, the thin side was that which 
was compressed: and as cast iron resists fracture by compression, with (on the 
average) about six times the force that it resists tension, the pillar seldom gave way 
by compression, and therefore bore nearly as much as it would have done if of equal 
thickness on both sides. 
If the thick side of the pillar had become that which was compressed, which, for 
what I can see, was as likely as otherwise, the strength would probably have been 
much decreased. But the circumstance of the thin side having been so often the 
compressed one, must apparently have arisen from some general cause. 
The difficulty of obtaining good uniform castings caused me to mention the circum- 
stance to Mr. Fairbairn, who gave orders that all the future pillars should be cast ver- 
tically, and in dry sand ; and this, together with great care, produced good castings, 
though the core could not always be kept in the centre towards the middle of the pillar. 
The greatest length of pillar to which the apparatus was adapted, was about seven 
feet six inches ; being a length half as much again as the greatest which was used in 
the experiments upon solid pillars. As, in the former experiments, the five-feet 
pillars had, by a mistake in the commencement, been made half an inch too long, my 
anxiety to avoid error, however slight, caused me to have the lengths of all the 
shorter pillars exact subdivisions of the longer ; this entailed upon all the future ex- 
periments fractional lengths, which have been introduced into the hollow cylinders, 
in order to make them exact multiples of the former. 
The length of the hollow pillars was therefore made 7 feet 6f inches ; and it was 
my wish (which was always seconded by my liberal friend Mr. Fairbairn) that their 
diameters should be as varied as possible, both for theoretical purposes and the ap- 
plication of their results to practice. There were difficulties, however, which militated 
MDCCCXL. 3 G 
