rge propeller diameter and reasonably low speed. The
diameter is the distance from end to end of the blades, which on the
largest propellers ranges from 6 to 8 feet. The larger the blade surface
the greater will be the volume of air displaced, and, following this,
the greater will be the impulse which forces the aeroplane ahead. In all
centrifugal motion there is more or less tendency to disintegration in
the form of "flying off" from the center, and the larger the revolving
object is the stronger is this tendency. This is illustrated in the many
instances in which big grindstones and fly-wheels have burst from being
revolved too fast. To have a propeller break apart in the air would
jeopardize the life of the aviator, and to guard against this it has
been found best to make its revolving action comparatively slow.
Besides this the slow motion (it is only comparatively slow) gives the
atmosphere a chance to refill the area disturbed by one propeller blade,
and thus have a new surface for the next blade to act upon.

Placing of the Motor.

As on other points, aviators differ widely in their ideas as to the
proper position for the motor. Wright locates his on the lower plane,
midway between the front and rear edges, but considerably to one side of
the exact center. He then counter-balances the engine weight by placing
his seat far enough away in the opposite direction to preserve the
center of gravity. This leaves a space in the center between the motor
and the operator in which a passenger may be carried without disturbing
the equilibrium.

Bleriot, on the contrary, has his motor directly in front and preserves
the center of gravity by taking his seat well back, this, with the
weight of the aeroplane, acting as a counter-balance.

On the Curtiss machine the motor is in the rear, the forward seat of the
operator, and weight of the horizontal rudder and damping plane in front
equalizing the engine weight.

No Perfect Motor as Yet.

Engine makers in the United States, England, France and Germany are all
seeking to produce an ideal motor for aviation purposes. Many of the
productions are highly creditable, but it may be truthfully said that
none of them quite fill the bill as regards a combination of the minimum
of weight with the maximum of reliable maintained power. They are all,
in some respects, improvements upon those previously in use, but the
great end sought for has not been fully attained.

One of the motors