igh speeds.

=Air Resistance.=--At slow speeds, the resistance of still air to
translation is small, but as the speed increases, the air resistance
increases rapidly and at the usual speed of the passenger automobile
on the road becomes a very considerable part of the total resistance
to translation. This factor has no significance in connection with
horse drawn vehicles, but is to be taken into account when dealing
with self-propelled vehicles at speeds in excess of five miles per
hour.

Many determinations of tractive resistance with horse drawn vehicles
have been made from time to time and these show values that are fairly
consistent when the inevitable variations in surfaces of the same type
are taken into account. Table 4 is a composite made up of values
selected from various reliable sources and Table 5 is from experiments
by Professor J. B. Davidson on California highways.

TABLE 4

AVERAGE TRACTIVE RESISTANCE OF ROAD SURFACES TO STEEL TIRED
VEHICLES

Surface Tractive force per ton

Earth packed and dry 100
Earth dusty 106
Earth muddy 190
Sand loose 320
Gravel good 51
Gravel loose 147
Cinders well-packed 92
Oiled road--dry 61
Oiled road--wet 108
Macadam--very good 38
Macadam--average 46
Sheet asphalt 38
Asphaltic concrete 40
Vitrified brick--new 56
Wood block--good 33
Wood block--poor 42
Cobblestone 54
Granite tramway 27
Asphalt block 52
Granite block 47

TABLE 5

TRACTIVE RESISTANCES TO STEEL TIRED VEHICLES[1]

----------+-----------------+-----------------+-----------+-----------
| | Condition | Tractive | Resistance
Test No. | Kind of Road | of Road | Total lb. | per ton lb.
----------+-----------------+-----------------+-----------+-----------
29-30-31 | Concrete |Good, excellent | 83.0 | 27.6
| (unsurfaced) | | |
[2]11-12 | Concrete |Good, excellent | 90.0 | 30.0
| (unsurfaced) | | |
26-27-28 | Concrete 3/8-in.|Good, excellent | 147.6