On the other hand Turner has recently made a very interesting point,9 namely, that the fourth dimension as actually treated by the mathematicians is not time itself, but time multiplied by a constant--the velocity of light.10 Without affecting the astronomical proofs of relativity at all, this simplifies our conceptions enormously. In ordinary everyday life time and space cannot be identical, any more than a yard can be identical with a quart. On what is known to physicists as the centimeter-gram-second system, distance is represented by $l$, mass by $m$, and time by $t$. Velocity is then distance divided by time, $l/t$, or as we say in English units, so many feet per second, and the fourth dimension may be expressed as time multiplied by velocity, $t \times l/t = l$. That is to say, it is simply distance, just like the other three dimensions. To say that time is the fourth dimension from this point of view, appears to us just as ridiculous as it would be to attempt to measure the velocity of a train in quarts. It is quite correct, however, although unusual, to speak of a given train as moving at a speed of 10 quarts per square inch per second, $l^3/l^2 t = l/t$. This would be equivalent to a velocity of 33 miles per hour. If I wish to give a complete dimensional description of myself in my four dimensions, I must give my length, my breadth, and my thickness, ever since I came into being, and also the course I have traversed through space since that time. This latter distance will be expressed in terms of a unit whose length is 186,000 miles, the distance traversed by light in one second. The distance which I travel through space annually is enormous, and very complex as to direction. It involves not merely my own motions as I cross the room, or take a train or steamer, but also those due to the rotation of the earth on its axis, its revolution round the sun, and the motion of the latter through the heavens. In general I travel, or in other words increase my length in the fourth dimension, by over 4,000 units a year. The fourth dimension accordingly, if this view is accepted, is simply a distance like the other three, and perfectly easy to understand.