200 Mr. Dunlop's Experiments on the Strength [Maney + 
ARTICLE VII. 
An, Account of some Experiments to ascertain the Strength of such 
Casi-Iron Shafts as are commonly used in Mill-Work, and what 
Proportion their Strength has. to their Diameters. By Mr. 
Dunlop, of Glasgow. 
WhuoEVER remembers the kind of machinery which we hadin 
this country about 25 years ago, will easily perceive, that, inde- 
pendently of other contrivances, it has been greatly improved by 
merely using cast-iron as a substitute for wood in the construc- 
tion of it. All our other improvements have been limited ; 
confined to particular machines ; but this, having increased the 
strength and durability of every machine, has improved the 
whole. 
That an improvement now so obvious and so important was 
not sooner observed and generally adopted is unaccountable ; 
and our practitioners in the mechanical arts, im place of taking 
merit to themselves for the discovery, seem rather to owe an 
apology to their country for the slowness of their proceedings. 
. The difference of expense was at one time a serious considera- 
tion, but did not continue long; for the price of timber advanced, 
whilst that of cast-iron remained about the same. The principal 
reason, perhaps, for our not using that metal in preference to 
wood was, its being easily broken, especially if in small pieces, 
by a jerk or a smart blow: wood, on the contrary, bends; and if . 
not greatly overstrained, continues to bend for a long time before 
it entirely gives way; and thus indicating its want of strength 
by its flexibility, gives time to have any part of a machine con- 
structed of wood either repaired or replaced. Cast-iron, on the 
contrary, gives no such indication of want of strength ; if over- . 
strained, it snaps in an instant, and endangers, perhaps, the 
lives of the people-employed to work the machinery. It was on 
this account that our mechanics were cautious in using it as a 
substitute for wood ; and when, in the course of practice, they 
had to judge of its strength, they were, as in similar cases, gene- 
rally led by their own experience and observation ; hut to get 
experience, required time; and hence the slow progress which 
was made in this improvement. Even Bolton and Watt seem to 
have had little confidence in the strength of cast-iron, particu- 
larly if subjected to ajerk, and continued for many years to make 
the beanis of their steam engines of wood. They employed wooden 
frames too for supporting their cylinders and crank-shafts; even 
their comnexion-rods and condenser cisterns were made of wood. 
Tt is no way astonishing then that our ordinary mechanics, many 
of whom had neither the science nor the practical skill of Bolton 
and Watt, were cautious. They had good reason, seeing these 
