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UNIFICATION OF METHODS IN TESTING MATERIALS. XXV. 
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Aetdine | 
in which a and £8 are constants to be determined by experiment, 
fis the sectional area, and / the length. For prisms of dissimilar 
cross sections, he proposes the following formula— 
= (0G) JZ 
where w is the circumference of the cross section. 
Prof. Martens after investigating these experiments has deduced 
the following formule for cast iron of the same quality but differ- 
ent cross sections. 
pie 
For rectangular sections ¢ = 4400 eri) id 5 
sides a and 6 
For square sections ... 7 = 4220 + —— 
For circular sections ... ¢ = 4320 + od 
For hollow sections 
outer and inner diametersa = 4230 + — Lae 
d and d, respectively 
ea Prisms and cylinders of similar geometrical form have 
Jf the same strength. 
Experiments by Rondelet on different kinds of stone proved 
that the total strength of similar geometrical test pieces varies as 
the square of the homologous sides. The strength of a prism of 
Square section is 0-98 times that of a cylinder of the same height 
and sectional area. In testing building stones it is necessary to 
cut it into the desired shape, otherwise the results will not be 
uniform. Chipping will affect the strength of small cubes more 
than large. All tests of this kind should be made on accurately 
Prepared specimens with the opposite surfaces exposed to crushing, 
rubbed to true parallel planes, and one of the bearings should be 
Spherical to ensure uniform distribution of stress. This applies 
to cements, concrete, bricks, etc., but the true planes may be pro- 
duced by applying a thin coat of plaster of Paris. 
