THE UTILIZATION OF ENERGY. 515 
periments. These, however, did not include speeds above 206 
meters per minute (about 74 miles per hour), and the work was done 
on a tread-power, so that there was no air resistance. At this 
moderate speed it is not probable that the latter factor would be a 
large one, but it is one which increases as the square of the 
velocity, so that at high speeds it constitutes the larger portion of 
the resistance. At high speeds, too, the muscles contract to a 
greater degree, thus decreasing their efficiency, and additional auxil- 
iary muscles are called into play, both directly and to aid the in- 
creased respiration. It is a matter of common experience that while 
a horse is able to travel for a number of miles consecutively at 6 to 
7 miles per hour, drawing a considerable load, he can maintain his 
highest speed for only a short time even without load, and does this 
only at the cost of largely increased metabolism. It is evident then 
that there is a limit beyond which an increase of trotting speed 
must increase the metabolism with comparative rapidity. 
Loap.—Supporting a load on the back while standing was found 
to increase the metabolism of the horse No. III approximately in 
proportion to the load—that is, the metabolism computed per unit 
of mass (horse+load) increased but very slightly. In locomotion 
with a load the metabolism is, of course, increased, since the load 
as well as the body of the animal must be lifted at each step. The 
increase over the metabolism at'rest and without load, both walking 
and trotting, was found in the case of Horse III to be somewhat 
greater (8-10 per cent.) than the increase in the mass moved 
(horse + load). 
After making allowance for this increase in the work of locomo- 
tion, the efficiency in ascent with a load was found to be unaffected 
by the latter; that is, the energy expended in lifting a unit of mass 
(horse+load) through a unit of distance remained substantially 
the same. Indeed the figure obtained (36.2 per cent.) is slightly 
higher than that without load (34.3 per cent). Interesting indi- 
vidual differences in the above particulars were, however, observed 
between Horse No. III and some of the other animals experimented 
upon, particularly Nos. II and XIII, which form the subject of a 
succeeding paragraph. 
Species AND Size or ANIMAL.—In ascending a moderate grade, 
the efficiency seems to be about the same in the horse and in 
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