410 
MR. HODGKINSON’S EXPERIMENTAL RESEARCHES 
against the execution of this desire. The external diameter of the pillars could not 
be reduced at pleasure; the internal must be large enough to admit a small hollow 
metallic tube, covered over with a thin stratum of loam, tied together with vegetable 
matter, the tube being perforated in every part with holes, and having its internal 
diameter large enough to allow the gas and steam to escape. 
Through these causes, the smallest pillars which we could cast of the above length 
were If inch external diameter, and 1^ internal. On the other hand, the experi- 
ments were bounded, in the largeness of the diameters, only by the power necessary 
to break the pillars. With Mr. Fairbairn’s apparatus (a single lever of which the 
greatest multiplier was 7 or 7i) I had never conceived it safe to apply more pressure 
than about 18 tons; and when an additional lever was added to this, the greatest 
weight used was 22 ^ tons. This was in one of the experiments on hollow pillars. 
Through these causes, the variation in the diameters of the longest hollow pillars 
was necessarily small, particularly in those with flat ends, of which the largest ex- 
ternal diameter was about 2f inches, this requiring upwards of 18 tons to break 
it. In this, the length of the pillar was 40 times the external diameter, and therefore 
might be considered as representing only the longest and slenderest pillars used in 
practice. In this matter, however, I was not without a resource, it having been dis- 
covered, in the early part of these inquiries, that the ratio of the strengths of solid 
pillars with rounded ends, to those with flat ones, was nearly constant, and about as 
one to three ; and this was the case with all pillars, from those of the greatest length, 
as compared with the diameter, down to a certain ratio of length, considerably shorter 
than we could command power to break, provided the ends of the pillar were flat. 
It was therefore determined to pursue the experiments upon these hollow pillars 
(putting solid hemispherical caps upon the ends), increasing the diameters by small 
degrees (the length being the same), till I could break them no further. 
In this way I was enabled to vary the external diameters from 1*74 to 3 - 36 inches, 
the former having sunk with 5711 lbs., and the latter with 50477 lbs., or 22^ tons. 
In the largest pillars of this series, experiments 15 and 16., Table VIII., the length 
was reduced to twenty-seven times the diameter. To pursue the matter further, it 
now became necessary to reduce the length of the pillars, because when that was 
done the internal diameter could be reduced, and thus the external might be cast so 
small that the strength of the metal in the pillar would be overcome by the power 
which I had at command. Several of these pillars were, at Mr. Fairbairn’s sugges- 
tion, turned outside and bored within, to make their dimensions as uniform as pos- 
sible. By adopting pillars of smaller size than before, I was now enabled to render 
them as short, compared with the diameters, as I wished, and thus obtain specimens 
of most of the varieties used in practice. (See Table X.) 
Tables VIII. and IX. contain the results of the experiments upon the longest 
hollow uniform cast-iron cylinders ; with an experiment or two in each, upon solid 
cylinders of the same length to compare with them. The cylinders were always cast 
