4 BULLETIN 114, U. S, DEPARTMENT OF AGRICULTURE. 
been developed primarily as a plowing engine, with belt work a sec- 
ondary consideration. Although it was actually superior to the steam 
tractor in the ways mentioned, it was nearly a decade before it 
developed sufficiently to prove this superiority and became a real 
competitor with the steam tractor. Most of its growth has Qccurred 
during the past 1 1 years, and in considering the rapidity with which 
it has been made it might at first appear that it must have been due 
to its superiority over both the horse and steam tractor as prime 
movers for the farm. As to its superiority over the steam tractor 
there is no doubt. The sales of steam tractors for farm work other 
than thrashing fell off as those of the gas tractor increased, and the 
steam tractor is seldom found to-day except in sections where suit- 
able fuel is cheap and convenient, thus giving it an advantage over 
the gas tractor. The decline in the number of steam tractors used 
for farm work is shown by the age distribution of those reported: 
One year old, 37; 2 years old, 65; 3 years old, 65; 4 years old, 88; 5 years old, 76; 
6 years old. 33; 7 years old, 25; S years old, 24. 
THE GAS TRACTOR AND THE HORSE. 
While the gas tractor has almost completely replaced the steam trac- 
tor, as has been stated, neither the steam nor gas tractor has affected 
the sale or use of farm horses to any great extent, (See Tables XXII 
and XXIII.) 
A careful study of the subject shows clearly that the rapid growth 
of the gas tractor was not due to its superiority over the horse, but to 
the fact that large tracts of unbroken prairie land were being opened 
up in the West and that sufficient horses were not available to break 
the ground and bring it under cultivation. Gas tractors could be, 
and were, manufactured in a much shorter time than it v\ T ould have 
taken to raise the necessary horses for this work. But as this new 
country developed, horses were rapidly imported, colts were raised, 
and more and more of the farm work was performed with horses. 
Quite frequently the tractor which had broken the prairie and brought 
it under cultivation was entirely replaced by them. 
A similar condition existed recently in Kansas. An epidemic 
diminished the number of farm horses in that State by thousands, and 
the number remaining was insufficient to perform the field work. 
Immediately hundreds of traction engines were shipped into the State 
to meet the power requirements. Whether these machines will 
retain the ground thus opened to them remains to be seen. Under 
similar conditions in other States they have not done so, indicating 
that they are either not as satisfactory as horses for farm work or 
are more expensive. 
The failure of the gas tractor to maintain its position as the prin- 
cipal prime mover in those sections where it was first introduced was 
