28 BULLETIN 174, IT. S. DEPARTMENT OF AGRICULTURE. 
It may be safely stated that on most soils, when they are in fit 
condition to be worked satisfactorily with horses, the modern gas 
tractor will cause no injurious packing. The slippage of the tractor's 
wheels in soft ground will probably be a more serious matter than the 
packing. 
COMPARISON OF DIFFERENT SIZES OF TRACTORS. 
Table XVIII was prepared in order to ascertain what influence 
the size of the tractor has on the results obtained with it. In this 
table the tractors working in the State of North Dakota have been 
shown separately from those in other States, and only figures furnished 
by men having two seasons' experience have been shown, for reasons 
already given. 
In tabulating the data by sizes of tractors it was found advisable 
to group them to a certain extent, in order to have a sufficient number 
in each class to give reliable, averages. They were accordingly 
arranged in five classes, as follows: (a) 8 to 14 horsepower, (b) 15 to 
19 horsepower, (c) 20 to 25 horsepower, (d) 26 to 30 horsepower, and 
(e) 40 horsepower and over. 
These classes were arbitrarily arranged so as to place a considerable 
number in each group and at the same time to keep the most common 
sizes in separate classes. The average rating of the tractors in each 
group is shown hi the table. Thus, the 8 to 14 horsepower class 
includes three common sizes: 8, 10, and 12, although there is not a 
very large number of any of these sizes. The 15 to 19 horsepower 
class consists almost entirely of 15 horsepower tractors. The 20 to 
25 horsepower class includes three common sizes: 20, 22, and 25, but, 
like the first class, none of these sizes has a very large number. The 
26 to 30 horsepower class contains 30-horsepower tractors almost 
exclusively. No machines with drawbar ratings between 30 and 40 
horsepower were reported, and the tractors in the fifth class are 
mostly 40-horsepower outfits, as very few larger sizes were reported. 
From this tabulation it would appear that the 15-horsepower 
tractors have a longer life than those of other sizes. The length of 
life seems to decrease slightly with the increase in size of tractors 
over 15 horsepower, while for the smaller sizes it is a little less than 
for the 15-horsepower tractors. The larger sizes of tractors lose more 
time per day than those of 15 horsepower or less, the loss increasing 
with the size of the tractor. The amount of special equipment 
required increases with the size of the tractor until the 30-horsepower 
size is reached. The amount of special equipment for the 40-horse- 
power tractor is less than for those of 30 horsepower. Previous 
tables have shown that the amount of work done by the 40-horse- 
power tractor, as well as the load drawn, is not commensurate with 
its rating, but no reason is known why the value of its equipment 
should be less than for the 30-horsepower tractor. 
