
510 
NATURE 
[ Oct. 26, 1871 

BALL ON MECHANICS* 
“PES object of this book is to “prove the elementary 
laws of mechanics by means of experiments ”—a 
method the exact opposite of that generally adopted. 
According to the usual method, a few very general prin- 
ciples are assumed as derived from experimental data, a 
group of intermediate principles is then obtained deduc- 
tively, by the aid of which the action of forces in particular 
cases can be analysed. The particular cases may be such 
as have an interest from their bearing on practical ques- 
tions, but they are only examples of a general method 
applicable to innumerable other cases. There are 
therefore two distinct objects for which mechanical 
experiments may be made—viz, either to verify the 
fundamental principles,.or to verify the deductions 
drawn in particular cases. Experiments of the former 
kind are absolutely essential to the existence of the 
science. Unless, for instance, the conditions of the 
action of the force of friction are determined by ex- 
periment, no deductions as to cases into which that 
force enters have any but a theoretical value. The same 
is true in all similar cases ; such questions as, whether 
quantity of matter is proportional to weight, whether 
gravity at a given station is sensibly a constant force, 
whether the elasticity of solid bodies follows Hooke’s law, 
and if so within what limits, can be answered by experi- 
ment only. Such questions, on the other hand, as the 
tension of a tie-rod under given circumstances, the rela- 
tion between the weights which keep a given lever at rest, 
aa 

the relation between the power and the weight in a block 
and tackle, the form of the surface of a revolving liquid, 
admit of exact answers by deduction from the proper data, 
and, of course, the answers may be tested by experiment. 
Such experiments clearly have a different object from those 
of the former class. They have, indeed, this in common, 
that experiments of the latter kind also serve to verify funda- 
mental principles, but they do so indirectly. It is, however, 
from the teacher’s point of view that their value will be found 
greatest. In teaching the elementary parts of mechanics 
perhaps the greatest difficulty experienced is to make the 
learner feel that the diagrams drawn on the black board re- 
present facts, that, for instance, the conclusion deduced from 
a triangle is really applicable to a crane. Put the ex- 
periment side by side with the deduction, and it will be 
seen that the experiment cannot fail to bring home to the 
mind of the learner that his reasoning relates to things 
and not merely to abstractions. 
Let CB (Fig. 1) represent the jib or strut, and AB the tie- | 
rod of a crane, the line AC being vertical. Let a weight P 
hang from A, and let it be required to determine the forces 
transmitted through the tie and the jib. P can be re- 
solved into two forces acting along BC and AB produced, 
and an inspection of the figure will show that these forces | 
bear to P the same ratio that the lines BC and AB bear 
to AC, and that the force along BC is a thrust, and that 
along AB atension. This analysis is perfectly general. 
% Experimental Mechanics : a Course of Lectures delivered at the Royal 
College of Science for Ireland. By Robert Stawell Ball, M.A. With Illus- 
trations. (London and New York: Macmillan and Co., 1871.) 


We will now give Mr. Ball’s experiment in illustration of 
the same question :—“‘A piece of pine BC, 3/ 6” Jong 
and 1” x 1” in section (Fig. 2)is capable of turning roundits 
support at the bottom B by means of a joint or hinge ; it 
is held up by a tie AC 3/ long, which is attached to the 
support exactly above the joint. AB is 1’ long. From 
the point C a wire descends, having a hook at the end, 
on which a weight canbe hung. The tie is attached to 
the spring balance, the index of which shows the strain. 
The spring balance is supported by a wire strainer, by 
turning the nut of which the length of the wire can be 
shortened or lengthened as occasion requires. This is 
necessary, because when different weights are suspended 
from the hook the spring is stretched more or less, and 
the screw is then employed to keep the entire length of 
the tie at 3’. The remainder of the tie consists of copper 
wire” (p. 29). Mr. Ball then goes on to notice that when 
a weight of 2olbs. is placed on the hook, the strain, as 
determined by the spring balance, is 6olbs., thus verifying 
the analysis of the case given above. 
As an example of an experiment of the former class we 
will take the following,—it is the form in which Mr, Ball 


























































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Fic. 2. 
gives Galileo’s experiment of dropping bodies from the 
top of the Tower of Pisa. The figure (Fig.3) isso perfect that 
it scarcely requires explanation. Solong as the current is 
in action, the horse-shoe G is magnetic, and a ball of iron 
F remains suspended from it. When the current is 
broken G is no longer magnetic and F falls. In this 
manner, by including the wire round both horse-shoes in 
the circuit, a ball of iron and one of wood, into which a 
flat-headed nail has been driven, can be kept suspended, 
and then by breaking the circuit they can be let fall at 
exactly the same instant, they are seen to reach the cushion 
| at the same instant, and are thus shown to fall through 
equal spaces in the same time. Mr. Ball describes and 
discusses the experiment at some length, and shows how 
it proves that at a given station the attraction of gravita- 
tion on different bodies is proportional to their masses. 
The above examples will give a better notion both of 
the contents and illustrations of the book than any long 
description. We may say, however, that the book con. 
tains a clear and correct exposition of the first principles 
of mechanics, and illustrates, by well-chosen experiments, 
all the points in the subject that can be fairly called 
elementary. The figures reproduce all the circumstances 
of the experiments with so much exactness that with 
