134 Professor H. S. HeJe-Skaw [Marcli 31, 



motion question turned on oue of speed, and the supporters of the 

 new departure were unable to make any headway for many years, 

 partly because the speed limit was put at between 3 and 4 miles an 

 hour, that is, the limit of a walking man. A few years ago the speed 

 of 12 miles an hour which, after a great struggle, was obtained, gave 

 place to 20 miles an hour. You can see from the diagrams which 

 Mr. Legros gave in a recent paper before the Institution of Mechanical 

 Engineers, and which have been brought up to date, how the speedier 

 self-propelled vehicle is leading to the disappearance of the horse, at 

 any rate in London, and the difficulty which most people seem to 

 feel is not how to get above the speed limit, but how to keep 

 within it, and the papers show, by a daily crop of sad examples, 

 how only too painfully easy it is not to do so. 



Nothing points more clearly to what I have indicated as the basis 

 of our instinctive desire for speed, as the fact that our measure of 

 speed is entirely relative. Thus 60 miles an hour would be a slow 

 speed for a motor-car on a racing track, as seen by the speeds of the 

 motor races at Brooklands last Saturday (April 25th), but this speed, 

 which would be even quite good along the open road to Brighton, 

 would be considered decidedly on the high side for motoring along 

 the Strand. Our ideas of what is slow and what is fast are largely 

 derived from habit, and particularly from surrounding conditions and 

 from our mode of estimation. For instance, we have been carried in 

 this hall during the last hour with the surface of the earth round its 

 axis a distance of about 600 miles. This speed would require a line 

 on our speed chart about as high as the dome of the hall to represent 

 it graphically. But if we judge the speed from observing the 

 apparent rate of motion of the moon and stars overhead, we could 

 never realise this. Far less could we realise by the change in the 

 seasons the speed at which we are travelling with the earth round the 

 sun, accomplishing a distance, as we do, of 540 million miles in 365 

 days, which represents roughly a distance of 60,000 miles per hour. 

 We have thus travelled together, since we came into this hall, a speed 

 of 60,000 miles. The line required on our chart for this speed would 

 be about as high as St. Paul's Cathedral. But these speeds fall far 

 short of those of certain heavenly bodies with which we are familiar, 

 such as the meteors, some of which are travelling at 160,000 miles an 

 hour, and the recent comet, which probably exceeded this speed one 

 part of its journey round the sun ; whereas the fastest speed which 

 man has, up to the present, been able to produce, even in a projectile, 

 amounts to between 2000 and 3000 miles an hour (the Krupp 10 "7 

 centimetre having a velocity of 3291 metres per second, and a 6-inch 

 Yickers, 3190 metres per second). The highest projectile speeds we 

 have attained are thus only about one-tenth of the speed at which 

 Jules Yerne fired M. Barbicane and his friends oif, in order to over- 

 come the earth's gravity and reach the moon, since the speed he 

 required was 12,000 yards per second, or 24,000 miles per hour. 



