Lecture I

The customary way is to recognize the smallest unit bearer of mechanical force by its effect, in answering this question: If such a particle brings another minute particle—say, a minute particle of matter weighing one gramme—into movement, there must be some force proceeding from the matter in the one, which brings the other into movement. If then the given mass brings the other mass, weighing one gramme, into movement in such a way that the latter goes a centimetre a second faster in each successive second, the former mass will have exerted a certain force. This force we are accustomed to regard as a kind of universal unit. If we are then able to say of some force that it is so many times greater than the force needed to make a gramme go a centimetre a second quicker every second, we know the ratio between the force in question and the chosen universal unit. If we express it as a weight, it is 0.001019 grammes' weight. Indeed, to express what this kind of force involves, we must have recourse to the balance—the weighing-machine. The unit force is equivalent to the downward thrust that comes into play when 0.001019 grammes are being weighed. So then I have to express myself in terms of something very outwardly real if I want to approach what is called “mass” in this Universe. Howsoever I may think it out, I can only express the concept “mass” by introducing what I get to know in quite external ways, namely a weight. In the last resort, it is by a weight that I express the mass, and even if I then go on to atomize it, I still express it by a weight.