pulse beats in the manner of Galileo, he compares it with the beats of an accurate astronomical clock, which he perceives by his ear while with his eye he notes the passage of the wire which supports the ball of the pendulum across an accurate mark, thus being obliged to use the senses of sight and hearing at the same time. He must then measure the length of the wire accurately as well as the diameter of the ball which hangs from it. Later on, as there will be difficulty in telling where the string or wire ends, more refined means must be adopted for defining and measuring its length. From the results of these measurements the student will by means of theory be able to calculate the result expressing the intensity of gravity and as he presumably knows the correct value, he will be under a certain temptation to so "doctor" his results as to make his work seem accurate. It is needless to say that such doctoring can never be tolerated and is totally incompatible with the character of a true scientist. The example which I have given shows the nature of almost all the work that is undertaken in the physical laboratory. In every experiment certain data are taken which enable us to give a numerical measure of the properties of certain bodies, or a statement of the numerical relations involved in phenomena. As an example we may take the question of the determination of the specific heat of bodies, that is to say, of the amount of heat required to heat a body through a certain range of temperature. For this purpose the body, say an iron ball, is heated to a certain definite temperature, let us say by being immersed in the steam of a boiler in which water is boiling. The ball is then dropped into a vessel containing a known quantity of water and the heat that it gives out in cooling is measured by the rise in temperature which the water undergoes. This apparently simple process is found to be attended with a great deal of difficulty. In the first place, the determination of the temperature of the ball, when in the steam boiler, is no easy matter. A thermometer immersed in the steam as near the ball as possible may not show exactly the temperature of the ball. Secondly, if the stem of the thermometer is entirely immersed in the hot steam the temperature shown would be different from that when only, the bulb of the thermometer is in the hot steam and the stem in the cool air. Thirdly, it will be difficult to transfer the ball from the hot steam to the cold water so quickly that it will not have lost some of its heat, which we want to measure, before it gets into the water. Fourthly, as soon as the temperature of the water in the calorimeter, as it is called, begins to rise the calorimeter begins to lose heat by radiation to outside bodies. In order to estimate this we must first study the laws of such radiation by allowing water previously heated to cool in the calorimeter and observe how rapidly its temperature falls. Finally, it is necessary to know accurately how much water was in the calorimeter, which is found by weighing, but during the
