readings and the known length of the side of the triangle formed by the points of the tripod.
5. Unit of Time.— The unit of time is the mean time second, which is the of a mean solar day. We employ the clock, regulated by the pendulum or the chronometer balance, to indicate seconds. The clock, while sufficiently accurate for ordinary use, must for exact investigations be frequently corrected by astronomical observations.
Smaller intervals of time than the second are measured by causing some vibrating body, as a tuning-fork, to trace its path along some suitable surface, on which also are recorded the beginning and end of the interval of time to be measured. The number of vibrations traced while the event is occurring determines its duration in known parts of a second.
In estimating the duration of certain phenomena giving rise to light, the revolving mirror may be employed. By its use, with proper accessories, intervals as small as forty billionths of a second have been estimated.
6. Unit of Mass.— The unit of mass usually adopted in scientific work is the gram. It is equal to the one-thousandth part of a certain piece of platinum, called the kilogram, preserved as a standard in the archives of France. This standard was intended to be equal in mass to one cubic decimetre of water at its greatest density.
Masses are compared by means of the balance, the construction of which will be discussed hereafter.
7. Measurement of Angles.— Angles are usually measured by reference to a divided circle graduated on the system of division upon which the ordinary trigonometrical tables are based. A pointer or an arm turns about the centre of the circle, and the
