24 THE FLIGHT OF BIRDS 
domestic Pigeons, and und that they account for 
about one-fifth, or even more than one-fifth, of the 
weight of the whole bird. Indeed, in one Wood- 
Pigeon they were equal to three thirteenths of the 
total weight—a little less than a quarter.* Hence 
the centre of gravity lies considerably below, though 
not so very far below, the shoulder joints. But the 
idea that it is good to have the centre of gravity of 
a flying machine low down is altogether a miscon- 
ception, a fact that can easily be put beyond all 
doubt by experiments with small gliders. Weight, 
placed low, tends to make the machine oscillate 
and even swing right round. It is best that it 
should be on a level with the supporting surface. 
In the case of the bird, the chief supporting surfaces, 
the wings, are always changing their relative position 
as they are raised or lowered, and, of course, as they 
move they to some extent raise or lower the centre 
of gravity. This must make voluntary adjustments 
for the purpose of recovering balance still more 
necessary. But Professor Marey has pointed out 
a curious fact which may make these oscillations 
less difficult for the bird to deal with. When the 
down-stroke takes place and the wings are lowered, 
the centre of gravity occupies a lower position in 
the bird, but the bird as a whole, except during 
very rapid flight, rises. With the up-stroke, on the 
other hand, there is a raising of the centre of gravity, 
but a lowering of the bird ; hence, though the bird’s 
* Legal and Reichel, in the Jahresberichte der Schlesischen 
Gesellschaft (1879), give 5.4; (=4192=more nearly 4 than 3) as 
the proportion of the total weight of the pigeon accounted for by 
the three pairs of breast muscles: But this would seem to bea 
mistake: Charadrius, according to them, comes next with 2. 
