142 
PRESIDENTIAL ADDRESS. 
before from general experience that unsound forgings are 
liable to be made from want of care and skill on the part of 
the workmen, and these defects sometimes proved very incon¬ 
venient and uncertain, and involving a dangerous risk of subse¬ 
quent breakage; but the matter was not properly understood till 
Tresca showed the real action that produced this unsoundness, 
and consequently the means of preventing its occurrence. 
The force of the blows in forging iron (that is, both the weight 
and the velocity of the hammer) must be proportionate to the 
mass operated upon; if this force is too light, the exterior portion 
gets drawn more than the core, leaving the ends of the forging 
concave, and causing unsound places or even hollows in the 
heart of the forging, such as a hole or “piping” extend¬ 
ing many inches up the centre of an axle from each end. 
On the other hand, if the blows are too heavy, the opposite 
effect is produced, and the central portion is elongated more 
than the exterior; the unsupported material in the centre 
being squeezed out at the ends in a convex form, whilst the 
surface is held back by the hold of the hammer and anvil 
faces, as in the experiments of discs compressed without 
supporting walls, shown in Fig. 8. 
It will be seen that these valuable experiments have an 
important bearing also upon other investigations, such as the 
geological changes that may have taken place in the under¬ 
lying portions of the earth’s crust, from the enormous 
pressure of the mass above them, or the lateral thrust pro¬ 
duced by the contraction of the earth’s crust—a pressure so 
enormous that it may possibly have been sufficient, if only a 
few miles, depth of superincumbent rock is considered, to 
reduce to a plastic and flowing state even the hardest of the 
rocks of which the earth’s crust is composed ; and the 
ingeniously varied forms in which Tresca’s experiments were 
carried out may assist in understanding the phenomena 
that are observed in some of the remarkable contortions of 
the strata forming the earth’s surface. 
BIRMINGHAM NATURAL HISTORY AND MICRO¬ 
SCOPICAL SOCIETY. 
PRESIDENTIAL ADDRESS. 
BY R. W. CHASE. 
(Continued from page 120). 
Changes in Local Faunas. 
A strong reason for the formation of Natural History 
Museums without delay is that in the course of a f'ewyears many 
