2 I 8 
James Small. 
between the glass and the fruit, but in the case of the dandelion the 
fruit rests on the glass on the tips on a few pappus hairs and a 
very small portion of the surface of the fruit body. Friction in 
such a case is reduced to a minimum. Another possible source of 
error is the adhesion of the fruit to the glass ; this might arise 
from a mucilaginous pericarp or from mucilaginous achenial hairs, 
both of which are absent in the dandelion, so that this source of 
error is eliminated. A third possible source of error is the 
assumption that the effect of the tangential component S is 
negligible but this assumption is justified by subsequent observation 
and calculations. 
With the dandelion the observed maximum wind which does 
not move the fruit is 1*01 m.p.h.(see table X) and in this case it can 
be taken as the velocity of wind with a pressure equal to the 
critical vertical component, -0026 gms. per sq. cm. 
Theoretical Conclusions. 
From these observed data various conclusions can be drawn, 
and the more interesting points will now be considered. 
Fall in Quiet Air. The first interesting point arises from the 
close similarity between the rate of fall in quiet air, *98 m.p.h. and 
the experimental value found for the critical vertical component, 
which is equivalent to 101 m.p.h. This is easily explained from 
the hydrodynamical point of view. In still air there is no lateral 
force, therefore there is no tilting of the fruit. The pappus surface 
is horizontal, so that there are two forces acting during the fall, the 
mass of the fruit and the pressure exerted by the air impinging on 
the pappus with a velocity equal to the rate of fall. The latter 
force is similar to that acting with a lateral wind but in this case it 
all acts in a vertical upward direction. The mass of the fruit acts 
downwards in a vertical direction. 
The fruit in quiet air rapidly acquires the terminal velocity of 
fall, then the upward force is equal to the downward force, the fall 
being due to the momentum gained before the terminal velocity is 
reached. The upward force due to a fall at the rate of -98 m.p.h. 
is equivalent to a wind with a velocity of ’98 m.p.h. The experimental 
value, F01 m.p.h., found is quite a close approximation, especially 
when it is remembered that, as Praeger mentions, there is some 
variation in fruits collected from different heads, although those 
from the same head are very similar. The rate of fall in quiet air 
is at least approximately equal to the velocity of the critical 
vertical component and is more easily and accurately measured by 
