festations of a few general principles. Regarding the universe as a machine such men are interested not so much in what it can do as in how it works and how it is made; and when they have succeeded, to their own satisfaction at any rate, in solving even a minute portion of this problem they experience a delight which makes the question "what is the value of hypothesis?" appear to them as irrelevant as the questions "what is the value of poetry?" "what is the value of music?" "what is the value of philosophy?"
Recent investigations on Electricity have done a good deal to unite various branches of Physics, and I wish this evening to call your attention to some of the consequences of applying the principle of the equality of action and reaction—Newton's Third Law of Motion—to some of these researches. According to this law the total amount of momentum in any self contained system, that is any system uninfluenced by other systems, is constant, so that if any part of such a system gains momentum another part of the system must simultaneously lose an equal amount of momentum. This law, besides being the foundation of our ordinary system of dynamics, is closely connected with our interpretation of the great principle of the Conservation of Energy, and its failure would deprive that principle of much of its meaning. According to that principle the sum of the kinetic and potential energies of a system is constant; let us consider a moment how we are to estimate the kinetic energy. To us the objects in this room appear at rest, and we should say that their kinetic energy was zero, but to an observer say on Mars, these objects would not appear to be at rest but moving with a considerable velocity, for they would have the velocity due to the rotation of the earth round its axis and also that due to the revolution of the earth round the sun; thus the estimate of the kinetic energy made by a Martian observer would be very different from our estimate. Now the question arises does the