630 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1911. 
Seward in 1847, or 64 years ago, still stands. In fact, a study of all 
the records of 25 distances shows that several of them remain un- 
broken after comparatively long periods, viz, from a quarter to half 
a century. 
Thus, so far as his own unaided powers of locomotion are con- 
cerned, man may be considered, for all practical purposes, to have 
reached long ago the limit of speed possibility. From earliest times, 
however, he has brought the muscular effort of other animals into 
his service, and has devoted his intellect toward improving their 
speed for his own uses. You will see graphically recorded in figure 1 
the speeds of all the Derby winners from the year 1856—i. e., for more 
than half a century. The average speed, which may be taken as 
somewhere above 30 miles an hour, has doubtless slightly increased, 
PEPER 
See ee es deren 
Sanne ees 
75 eo. = 8B 
3 1890 1895 i900 1905 i910 
MILES PER MOUR. 
fEAR 
Fig. 1.—Derby winners for 55 years. 
but it will be seen from the dotted line which has been drawn at the 
top of the maximum speeds what comparatively little increase has 
been obtained for an expenditure of the many millions represented 
directly and indirectly in the training and breeding of these horses, 
and it may be reasonably assumed that here again the limit has been 
reached for the fleetest animal, by the aid of which man can increase 
his speed of locomotion by using muscular power other than his own. 
What, then, are the physical reasons for this limitation? It is 
not due to the chief cause, which we shall see later puts a practical 
limit to very high speeds in mechanical locomotion, namely, the 
resistance of the atmosphere. Neither is it due to the effective work 
done in movement, since with a body moving along a level plan—.e., 
at a constant distance from the earth’s center—this effective work is 
