f ordinary dimensions.
Microscopic forces, so to speak, have to be observed, and therefore
all the causes of errors have to be avoided which would be unimportant
in most other physical researches. It is known that Cavendish was the
first who succeeded by means of the torsion balance in effecting
fairly precise measurements. This method has been again taken in hand
by different experimenters, and the most recent results are due to Mr
Vernon Boys. This learned physicist is also the author of a most
useful practical invention, and has succeeded in making quartz threads
as fine as can be desired and extremely uniform. He finds that these
threads possess valuable properties, such as perfect elasticity and
great tenacity. He has been able, with threads not more than 1/500 of
a millimetre in diameter, to measure with precision couples of an
order formerly considered outside the range of experiment, and to
reduce the dimensions of the apparatus of Cavendish in the proportion
of 150 to 1. The great advantage found in the use of these small
instruments is the better avoidance of the perturbations arising from
draughts of air, and of the very serious influence of the slightest
inequality in temperature.

Other methods have been employed in late years by other experimenters,
such as the method of Baron Eoetvoes, founded on the use of a torsion
lever, the method of the ordinary balance, used especially by
Professors Richarz and Krigar-Menzel and also by Professor Poynting,
and the method of M. Wilsing, who uses a balance with a vertical beam.
The results fairly agree, and lead to attributing to the earth a
density equal to 5.527.

The most familiar manifestation of gravitation is gravity. The action
of the earth on the unit of mass placed in one point, and the
intensity of gravity, is measured, as we know, by the aid of a
pendulum. The methods of measurement, whether by absolute or by
relative determinations, so greatly improved by Borda and Bessel, have
been still further improved by various geodesians, among whom should
be mentioned M. von Sterneek and General Defforges. Numerous
observations have been made in all parts of the world by various
explorers, and have led to a fairly complete knowledge of the
distribution of gravity over the surface of the globe. Thus we have
succeeded in making evident anomalies which would not easily find
their place in the formula of Clairaut.

Another constant, the determination of which is of