20 REPORTS ON THE STATE OF SCIENCE.—1918. 
and rhymer to a radius of 2 mm., the material between the hole and the 
bottom being cut away with a saw. 
An intermediate size has been proposed and used extensively, and 
the authors have adopted it as well as the two former sizes ; it is 160 mm. 
long by 20 by 20 mm. in cross-section, with a similar notch to the 30 by 
30 mm. piece, but only 5 mm. deep. 
In addition to notched bar tests the authors have made tension tests 
on specimens without notches. 
(a) + in. diameter and 24 in. gauge length, broken with a single blow 
in a small Charpy machine. 
(b) 10 mm. diameter and 100 mm. gauge length, broken with a series 
of blows in a Martens drop-hammer machine. 
The advantage of the circular notch over the acute triangular notch 
is that it can be made and reproduced accurately in a regular manner, 
so that the conditions of the test in so far as the notch is concerned are 
kept as constant as possible. 
The disadvantage is that more energy is consumed during the period 
when the bar undergoes deformation, before a crack is developed at the 
notch, than occurs with the acute triangular notch. The resilience ex- 
pressed in regard to the area of the cross-section through the notch is 
greater with circular than with triangular notches, and increases with 
the radius of the circular notch. At the same time the relative results 
obtained in grading material with circular or triangular notches do not 
vary to any great extent. If, however, the work done during the two 
periods of deformation and fracture could be separated and accurately 
determined, the triangular notch would undoubtedly supersede the 
circular notch, as the results of the test referred to the area of the cross- 
section through the notch would more accurately express the resilience. 
As to the advantages of the small test-pieces, 10 by 10 mm., over 
the larger, 30 by 30 mm.:—A larger number of pieces can be obtained 
from a given piece of material, and local defects more clearly exposed. 
Again, the selection of the test-pieces after a microscopic examination 
of a suitably prepared and etched surface revealing any heterogeneity 
iS more completely accomplished with the smaller test-pieces. The 
larger test-pieces enclose more or less any local defects, and it is not so 
eisy or convenient to obtain sufficient specimens. 
The smaller test-piece can be broken in a smaller machine. 
Impact-testing Machines.—All impact-testing machines should be 
designed in such a manner that the actual energy required to fracture 
the test-piece in one blow may be accurately determined. They should 
be calibrated from time to time, and the losses of energy due to friction 
and other causes should be accurately determined. The machines should 
be kept in good order and handled carefully. The machines used in the 
various tests made by the authors are sufficiently known, so that a detailed 
description is unnecessary, but it may be necessary to state the following 
particulars :— 
A. The Guillery Machine-—This machine consists of a fly-wheel 
21 in. diameter, carrying a knife with striking edge 2 mm. radius. 
The knife-edges supporting the test-piece are spaced 40 mm. apart, 
and are rounded to 2 mm. radius. (Vide Fig. 7.) 
The maximum velocity of impact is equivalent to a fall of 4°8 metres 
and the corresponding energy is 60 kilogrammetres. (K.G.M.) 
