During the nineteenth century, the path of scientific discovery might almost be represented by a vertical line. Never before was such rapid and marvelous progress made. The releasing of the mind from the oppressive restrictions of earlier conservative ages liberated the intellectual energies of mankind. A new idealistic philosophy supplanted that of an earlier period and universal attention was given to science and material things. Amidst these changes social science was devolved, and, with it, the study of psychology. But it was the physical sciences which most felt the stimulus of the new rationalistic spirit. The relationships between physical magnitudes are established by measurements. When these are accurately ascertained, questions regarding their variable functions can be solved by mathematical principles. Physics is thus linked with mathematics through measurements. The more science advances, the greater is the accuracy needed in physical measurements. The strictness and clearness of experimentation which has been attained in physics has given birth to a science of measurement, which has its own instruments, rules, methods, and formulæ. Measurement of length is one of the bases of physics. It is a relative operation carried out by comparing the length of one body with that of another. Standards of length are preserved by a Bureau of Weights and Measures in most countries. Delambre, a French authority on the decimal system of measures, taught at the beginning of the nineteenth century that magnitudes as small as the hundredth of a millimeter are incapable of observation. The International Bureau of Weights and Measures now guarantees to determine two or three ten-thousandths of a millimeter. So much has the science of measurement progressed in a century. The undulations of light rays are used for determining standard lengths. Michelson and Benoit measured a standard length of ten centimeters, in 1894, in terms of the wave lengths of the red, green, and blue radiations of cadmium, and then in terms of the French standard meter. These experiments yielded very accurate results. The measurement of mass is another important base of physics. Mass is the quantity of matter in a body and the action which gravity exerts on mass is called weight. Weight does not depend entirely upon mass, but also upon the position of the body weighed, because when the body is weighed in one place and reweighed in another, there will be a difference in the force of gravity due to change of latitude and of altitude. National standards of mass have been made of alloys of iridium and platinum. Many remarkable measurements of time, temperature, and physical constants were carried out during the century. High and low temperature charts were completed, showing temperatures in the air, the earth, and the sea. Instruments and methods were devised for measuring any temperature whether of high furnace gases or low freezing mixtures.