340 
NATURE 
| dugust 13, 1885 
while a general review of the subject of length measure- 
ment, in which other instruments and tools, including the 
cathetometer microscope, are described, concludes the 
chapter. 
This brief review of the first is sufficient to show the 
system on which each of the eight chapters is put 
together. 
In the second chapter on angular measurement the 
circular Vernier, the mirror and scale, and the spirit level 
of course form the subject of lessons. There is also a 
lesson on that simple and easily-constructed instrument 
of M. Cornu, the optical lever. 
The chapter on the estimation of mass is very complete, 
for, besides an explanation of the theory and use of the 
balance, there is a page headed “ precautions in weighing,” 
a copy of which might well be placed on the wall above 
every balance in a laboratory ; there is an excellent para- 
graph on the sensibility of the balance, with a diagram 
showing the observed sensibility of an Oertling, a Bunge, 
and another short beam balance. There is, lastly, a 
lesson on the errors of weights, in which instructions are 
given for testing a set of weights. 
In the chapter on measurement of area and volume a 
large amount of space is given to an explanation of the 
Amsler planimeter. This beautiful little instrument, as 
is well known, gives the area of a figure round which its 
point is traced. In anew edition it is to be hoped that 
the new “precision” planimeters which in accuracy and 
some other respects are superior to that of Amsler, will 
be described. 
In Chapter V., on the determination of density, are to 
be found full instructions for finding the specific gravity of 
solids and liquids by a host of methods. The corrections 
for buoyancy are carried to such an extent that account 
js taken of the latitude and the height above the sea- 
level in calculating the density of the air from the baro- 
meter reading ; further, the effect of moisture in lightening 
the air is guarded against. The hydrometers of Fahren- 
heit, Baumé and Twaddle are described, and in- 
structions are given for making them. The exact de- 
termination of the density of a gas, being a problem of 
great difficulty, is considered unsuitable for imitation in 
the laboratory ; however, an outline of Regnault’s method 
is given. 
The chapter on elasticity, tenacity, and capillarity 
differs from others in the book in that the theory of the 
subject is given at length, as well as instructions for per- 
forming experiments in the laboratory. 
The chapter on the determination of atmospheric 
pressure contains a full account of the method of filling 
and using a standard mercurial barometer. The aneroid 
barometer is not mentioned. 
The last chapter, on time, gravitation, and moments 
of inertia, is purely mechanical. The difference between 
the sidereal, solar, and mean solar days is explained, but 
instructions are not given for taking a transit. Clocks, 
chronometers, stop-watches, the water-clock, and the 
chronograph of Hipp, in which a reed vibrating 1000 
times a second replaces the pendulum of a clock, are 
briefly described. 
The determination of g by Borda’s and by Kater’s 
method is given. Several forms of electro-chronograph 
are described—among them one in which a primary 
circuit is broken at the beginning and end of the interval 
to be measured, while the induced currents cause a spark 
to pass between the style of a tuning-fork and a smoked 
drum, so that the number of waves between the two dots 
produced by the sparks measures the time. 
As has been already said, completeness and attention 
to details are apparent in every chapter of the first 
volume, while the names of the authors are sufficient 
as a guarantee of accuracy. The only cause for regret 
is the fact that the public has to wait for the two volumes 
on real physics, for those who read the first, which deals 
mainly with measurements of geometrical and mechanical 
properties, and which is therefore essentially an intro- 
duction, are likely to be impatient to see the series 
completed. 
OUR BOOK SHELF 
The History of a Lump of Gold from the Minz to the 
Mint. By Alexander Watt. (London: A. Johnson.) 
THE author has endeavoured to treat his subject so 
as to interest general readers, but he might have 
spared them such moralisations, suggested by the 
word “ gold,” as “With what silent rapture we re- 
ceive it as our own, and how different is the feeling 
when it comes into our hands merely to convey to 
another.” The compilation of facts connected with the 
history of gold and its manufacture into coin has, how- 
ever, been carefully done. Considering that the super- 
structure of modern chemistry was built up on the labours 
of the “early alchemists,” we object to their being de- 
scribed as “ those remarkable imposters,” and indeed the 
quotations from the writings of the early chemists which 
are given abundantly prove their claim to more respectful 
treatment. The metallurgy of gold is dealt with in the 
most slender way, but the chapter relating to the opera- 
tions of coinage is more satisfactory, and is confessedly 
an abstract of a series of Cantor lectures recently deli- 
vered by the chemist of the Mint. 
The important question of the amount of gold actually 
in circulation has not been lost sight of, and the author 
sums it up by quoting the following passage :—“ The 
amount of gold actually in circulation is estimated to be 
100,000,000/., but the coinage returns show that the 
amount of sovereigns and half-sovereigns issued since 
1816, when their coinage began, is 247,521,429. What, 
then, has become of the one hundred and forty-seven 
millions not in circulation?” No doubt a considerable 
proportion has been exported never to return, but we do 
not think, with the author, that the operations of manu- 
facturing goldsmiths and jewellers would account for a 
very large proportion of the deficiency. 
There are some remarkable slips in the printing. For 
instance, the well-known historian of the coinage is 
called the Rev. Rogers Rudling, and Sir John Pettus 
appears as Petters ; but viewed as a whole, the work may 
be commended as tending to disseminate information 
respecting the precious metal which it is desirable should 
be widely known. 
Magnetism and Electricity. By W.G. Baker. 
Blackie and Son.) 
WITH the multitude of elementary text-books on mag- 
netism and electricity already existing the production of 
a fresh one might well have seemed an unnecessary task. 
Nor is there anything in the little book now before us in 
the least degree new, either in matter or in arrangement. 
So far as it goes, however, it is quite satisfactory. It 
consists of 143 pages, and in this space the author has 
managed to give in a clear manner an account of so 
much of the subject as might reasonably be put before a 
(London : 
