in any direction it pleases, so long as the momentum lasts. “ If 
when sailing against the wind, the inclination of the body is such 
that the upward pressure of the wind against his wings and body 
just balances the force of gravity, his momentum alone acts, and 
he sails straight in the ‘ winds eye.’ If he wishes to ascend, he in- 
c ines his body more to the horizon by means of his head and tail.”# 
i hero is still another feat performed by some birds, requiring 
pater skill, and power of wing more wonderful, even, than that 
just descnbed, if we bear in mind the diTGci fomard impulse given 
at every stroke of thd wing in ordinary ilight. This is the power 
of “hovering/’ or remaining suspended in the air without prorrres- 
sing hung like a speck in the clouds, as we often see the kestrel 
when hunting over the fields in search of its prey. The mode in 
which this IS accomplished, is thus graphically described by the 
Duke of Argyll Of course, if a bird, by altering the a.xis of 
ts own body, can direct its wing-stroke in some degree forwards. 
It will have tlie effect of stopping instead of promoting progression! 
But in order to do this, it must have a superabundance of 
sustaining force, because some of this force is sacrificed when the 
stroke IS off the perpendicular. Hence it follows, that birds so 
heavy as to require the whole action of their wings to sustain them 
at all, can never afford this sacrifice of the sustaining force, and, 
except for the purpose of arresting their flight, can never strike 
except directly downwards -that is, directly against the opposing 
orce of gravity. But birds with superabundant power, and lon<^ 
sharp wings, have nothing to do but diminish the length of stroke'" 
and direct it ofiF the perpendicular, at such an angle as ^vill bring 
all the forces bearing upon their body to an e.xact balance, and 
they will then remain stationary at a fixed point in the air. 
“They are greatly assisted in this beautiful evolution by an 
adverse current of air; and it will always be observed, that the 
kestrel, when hovering, turns his head io wind, and hangs his 
whole body at a greater or less angle to the plane of the horizon. 
hen there is no wind, or very little, the sustaining force is kept 
up by a short, rapid action of the pinions, and the long tail is 
spread out like a fan, to assist in stopping any tendency to onward 
motion. AVhen there is a strong breeze, no flapping is required at 
aU— the force of the wind supplying the whole force necessary to 
♦ Ibis, 1S65, p. 296. 
