226 
MR. W. H. BARLOW ON THE RESISTANCE OF FLEXURE. 
ment was, to bring- the pins up by means of the screw against the side of the holes 
witli a certain degree of pressure, which, with a little practice in using the instrument, 
was attained with considerable accuracy. 
Two beams were employed in order to avoid errors which might arise from acci- 
dental irregularities in the metal. The head of the adjusting screw was graduated 
to 100 divisions, and the screw had 43-9 threads to the inch, so that one division was 
equal to 4 -^ 9 ^th of an inch. 
The measurements were, in all cases, taken by the outsides of the pins of the mea- 
suring instrument ; and when the instrument read zero, the actual distance of the 
outer sides of the two pins was - 4 - 3 %^ inches, so that the constant number 51661 
being added to the micrometer readings gives, in each case, the total distance in 
terms of of an inch. The form and dimensions of these beams are given in 
Plate Xlil. 
The measurements were taken four times in each position of the beam, and the 
error of measurement did not generally exceed from one to two divisions; but if in 
the four observations an error amounting to more than four was found, it was cor- 
rected by remeasurement. 
The numbers given in the following Tables are the micrometer readings, and the 
means of four observations in each case. In these experiments more than 3000 mea- 
surements were taken ; but to avoid unnecessary figures, only the more prominent 
results are given. 
Table No. I. contains the measurements of the centre division of the first beam 
under eight different conditions. 
Table No. II. contains similar measurements of the second beam.' 
In the first experiment it was found that, when the beam was inverted, the mea- 
suring instrument appeared to bear upon a different part of the holes, so that a direct 
comparison between the distances, in the beam erect and inverted, cannot be made 
with the same accuracy as the comparisons of different strains upon the beam when 
in the same position. The first beam had been subjected to strain for the purpose of 
testing the measuring instrument previous to these experiments being made ; but the 
second beam had not ; and it will be seen that the effect of the strains in the latter 
case caused a permanent lengthening of the beam. The same strain was frequently 
applied afterwards, but I could not observe any increase of this effect. There was 
certainly a further apparent lengthening of both beams; but I ascertained that this 
arose from a slight wearing of the working parts of the measuring instrument, from 
the great number of measurements taken. In both experiments the beam was 
measured, first, in an erect position ; and secondly, inverted ; but in the Tables, the 
measurements of the same parts of the beam are plaeed opposite each other, so that 
they may be compared throughout with greater facility. 
