ON STRENGTH AND PROPERTIES OF CAST IRON. 363 
of support. These ends were slides which enabled the straight 
edge to be raised or lowered at pleasure. In this manner it 
was easy to bring it down to touch in the slightest manner a 
piece of wood tied upon the middle of the bar. A candle was 
then placed at the side of the bar opposite to where the ob- 
server stood, by the light of which, distances extremely minute 
could be observed. Should it be asked why this had not been 
noticed before, the answer of the writer would be, that most 
experimenters have used bars shorter in proportion to their 
_ depth than are here employed, and therefore the set was much 
less obvious than here ; and in deep bars or beams it is almost 
imperceptible till the weight laid on is considerable. 
From what has been stated above, deduced from experi- 
ments made with great care, it is evident that the maxim of 
loading bodies within the elastic limit has no foundation in 
nature; but it will be considered as a compensating fact, that 
materials will bear for an indefinite time a much greater load 
than has hitherto been conceived. 
When a body is subjected to a transverse strain some of its 
particles are extended and others compressed ; I was desirous 
to ascertain whether the above defect in elasticity arose from 
tension or compression, or both. Experinfents 4 and 5 show 
this; in these a section of the casting, which was uniform 
c 
throughout, was the form ©] . During the experiments the 
a b 
broad fiat part a 6 was laid horizontally upon supports; the 
vertical rib ¢ in the latter experiment being upwards, in the 
former downwards. When it was downwards the rib was ex- 
tended, when upwards the rib was compressed. In both cases 
the partab was the fulcrum; it was thin and therefore easily flex- 
ible, but its breadth was such that it was nearly inextensibleand 
_ incompressible comparatively with the vertical rib. We may 
_ therefore assume that nearly the whole flexure which takes place 
in a bar of this form arises from the extension or compression 
_ of the rib, according as it is downwards or upwards. In ex- 
_ periment 4 we have extension nearly without compression, and 
_ in experiment 5 compression almost without extension. These 
experiments were made with creat care, and their results are 
generally in accordance with those from two others alluded 
_to above, but not inserted. They show that there is but little 
difference in the quantity of the set, whether it arises from 
_ tension or compression. 
_ The set from compression however is usually somewhat less 
4 than that from extension, as is seen in the commencement of 
_ the two experiments, and near the time of fracture, in that 
