In order to measure from an event to an event with a clock you must make an adjustment of the clock analogous to orienting a scale along the line . What is this analogous adjustment? The purpose in either case is to bring both and into the immediate neighbourhood of the scale or clock. For the clock that means that after experiencing the event it must travel with the appropriate velocity needed to reach the locality of just at the moment that happens. Thus the velocity of the clock is prescribed. One further point should be noticed. After measuring with a scale from [Pg 135] to you can turn your scale round and measure from to , obtaining the same result. But you cannot turn a clock round, i.e. make it go backwards in time. That is important because it decides which two sides are less than the third side. If you choose the wrong pair the enunciation of the time proposition refers to an impossible kind of measurement and becomes meaningless. You remember the traveller (p. 39) who went off to a distant star and returned absurdly young. He was a clock measuring two sides of a time-triangle. He recorded less time than the stay-at-home observer who was a clock measuring the third side. Need I defend my calling him a clock? We are all of us clocks whose faces tell the passing years. This comparison was simply an example of the geometrical proposition about time-triangles (which in turn is a particular case of Einstein’s law of longest track). The result is quite explicable in the ordinary mechanical way. All the particles in the traveller’s body increase in mass on account of his high velocity according to the law already discussed and verified by experiment. This renders them more sluggish, and the traveller lives more slowly according to terrestrial time-reckoning. However, the fact that the result is reasonable and explicable does not render it the less true as a proposition of time geometry.