Page:Elementary Text-book of Physics (Anthony, 1897).djvu/18

4 attempt to change that state. This property is called inertia. It must be carefully distinguished from inactivity.

Another essential property of matter is impenetrability, or the property of occupying space to the exclusion of other matter.

We are almost constantly obliged, in physical science, to measure the quantities with which we deal. We measure a quantity when we compare it with some standard of the same kind. A simple number expresses the result of the comparison.

If we adopt arbitrary units of length, time, and mass (or quantity of matter), we can express the measure of all other quantities in terms of these so-called fundamental units. A unit of any other quantity, thus expressed, is called a derived unit.

It is convenient, in defining the measure of derived units, to speak of the ratio between, or the product of, two dissimilar quantities, such as space and time. This must always be understood to mean the ratio between, or the product of, the numbers expressing those quantities in the fundamental units. The result of taking such a ratio or product of two dissimilar quantities is a number expressing a third quantity in terms of a derived unit.

4. Unit of Length.—The unit of length usually adopted in scientific work is the centimetre. It is the one hundredth part of the length of a certain piece of platinum, declared to be a standard by legislative act, and preserved in the archives of France. This standard, called the metre, was designed to be equal in length to one ten-millionth of the earth's quadrant. The operation of comparing a length with the standard is often difficult of direct accomplishment. This may arise from the minuteness of the object or distance to be measured, from the distant point at which the measurement is to end being inaccessible, or from the difficulty of accurately dividing our standard into very small fractional parts. In all such cases we have recourse to indirect methods, by which the difficulties are more or less completely obviated.

The vernier enables us to estimate small fractions of the unit of length with great convenience and accuracy. It consists of an