- 118 NATURAL HISTORY OF WASPS. 
as the sails of a ship, to the direction of the current 
of air. 
Thus far of the passive instruments of flight, of 
the means by which a heavy body can be supported 
in the air, without any greater effort of its own than 
putting itself mto the proper position to be helped. 
Turning now to the AcTIVE elements, we find two 
most important principles clearly laid down with 
regard to the use of the means of propulsion. First, 
the propelling power of a wing, as measured by the 
rapidity of flight, the strength being the same, is 
proportioned to its length rather than to its width. 
A fact which is turned to practical use in the con- 
struction of the fans of screw-propellers. And next, 
for the fans or wings to work to advantage, they must 
strike on dead air or dead water, that is on air or water 
which has not already been set in motion by, and is 
slipping away from, them. The distance to which 
the air is set in such motion by the wings of a bird 
is very short. For instance, birds do not ruffle the 
surface of the water, however closely they may fly 
over it. But within this distance a bird flies under 
as great disadvantages as a swimmer who is strug- 
glng agaist the stream with the water slippmg 
away from his stroke. Bearing in mind that a bird 
moves in a current, as it were, falling straight down 
to the ground, and that all blows given directly 
downwards are given im air slipping away from the 
wing, we see at once the reason why none but the 
very strongest flyers can ascend vertically, but must 
soar round and round, towermg up as it is called. 
When birds or insects try to poise themselves in the 
air, the rapid fluttermg of their wings, under these 
