354 : SEVENTH REPORT—1837. » Ne 
another, namely, that the power of bearing an horizontal im- 
pact from a given weight is measured by the strength of the 
beam multiplied by its ultimate deflection. This last assump- 
tion supposes that all cast-iron bars of the same dimensions 
in our experiments are of the same weight, and that the de- 
flection of a beam up to the breaking weight would be as the 
pressure. Neither of these is true, they are only approxi- 
mations ; but the difference in the weights of cast-iron bars of 
equal size is very little, and taking them as the same, it may 
be inferred from my paper on Impact upon Beams (Fifth Re- 
port of the British Association), that the assumption above 
gives results near enough for practice. 
After the following tables, therefore, there will always be 
given a summary of the strengths and deflections, reduced to 
what they would have been supposing the bar to be of the 
exact size of the model; and attached to these there will be 
the other values mentioned above, representing the power of 
the beam to bear impact. 
The modulus of elasticity is set down that it may serve as 
a measure of the comparative stiffness of the irons. It is given 
in pounds per square inch. 
The ultimate deflection attached to each experiment was 
derived from the results last obtained, and as these results 
were usually more numerous than those set down, the deflec- 
tion cannot often be calculated from those which remain, but 
is nearer to the truth than those which might be obtained from 
the remaining ones. 
In all the future experiments, the bars were cast 5 feet 
long, and were supported on props 4 feet 6 inches asunder, 
except it is otherwise mentioned, which will only be found in 
two cases. 
In the prosecution of the experimental part of this research, 
it gives me great pleasure to acknowledge the efficient manner 
in which my views were carried into execution by Mr. John 
Patchett, an intelligent pupil of Mr. Fairbairn. 
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