came very late, perhaps not much more than one hundred years ago. The first physical phenomena to be studied were, undoubtedly, those of day and night, the rising and setting of the sun and moon, and the changes of the seasons. We know very well that a people as highly cultivated as the Greeks, although they were deeply interested in natural phenomena, had an extremely small knowledge of the laws of nature and had not learned how to investigate them. Although they possessed an excellent knowledge of geometry, they had not the slightest idea of the nature or laws of motion, whether celestial or terrestrial, and with the exception of the properties of the lever and of liquids at rest, known to Archimedes, their knowledge of physics was almost a blank, and yet their great philosopher, Aristotle, dominated science until the sixteenth century of our era. It was at this time that the dawn of modern physics took place with the beginning of the experimental study of nature by Galileo. We must remember that Aristotle suffered not so much from lack of knowledge as from lack of appliances. What might he not have discovered had he possessed a thermometer, a telescope or even a clock! Nevertheless, Galileo did not possess these simple instruments, but he went to work to make them possible. He invented the telescope and thus made possible the searching of the mysteries of the heavens. Although he had no clock, he studied the motions of the pendulum, formed by the great lamp in the baptistery at Pisa, by comparing the time of its swing with the number of beats of his pulse, thus making possible the application of the pendulum to clocks by Huygens. As a contrast of Galileo's method with that of the Greeks may be cited his experiment of dropping a light and a heavy body from the top of the leaning tower of Pisa and showing that both fell to the ground at the same time, instead of believing, as the Greeks had done from reasoning without experiment, that the heavy body falls the faster. By careful study of the motions of a ball rolling down an inclined plane, Galileo was able to enunciate the precise law of falling bodies; that their acceleration is uniform, that is, that in equal times their velocity increases by equal amounts. The way was thus prepared for Newton, who in the next century established the connection of all forces with the accelerations produced by them and was able to enunciate the laws of motion, both terrestrial and celestial, in a form that has not been improved upon to-day, constituting one of the most magnificent triumphs of the human intellect. Passing on rapidly we find at the beginning of the nineteenth century the phenomena of electricity beginning to attract the attention of investigators, while those relating to light had already made substantial progress. All this time there had been nothing that could properly be called a physical laboratory. Discoveries had been made by individual inquirers working generally in such rooms as they had in their own houses with the most meager facilities. We all remember how Newton bored a hole
