184 Natural and Artificial Flight. (April, 
when the animals were progressing at a high speed, being 
opened out or unravelled to form first a looped, and then a waved 
track.* The following quotation from his memoir will explain 
the relation :—‘‘ The water and air are acted upon by curve 
or wave pressure, emanating in the one instance from the 
body of the fish, and in the other from the wing of the bird 
—the reciprocating and opposite curves into which the trunk 
and wing are thrown in swimming and flying, constituting, 
in reality, a mobile helix or screw, which, during its action, 
produces the precise kind and degree of pressure adapted to 
fluid media, and to which they respond with the greatest 
readiness.” He also contrasted the screw formed by the 
wing with that at present employed in navigation; and 
showed that the latter, which is rigid, cannot be compared 
in point of efficiency with the former, which is flexible and 
elastic. ‘The rigid screw of the ship is made to revolve, the 
one blade following the other in rapid succession, and all 
striking at a given angle which never varies. One blade, as 
a consequence, virtually performs all the work. From the 
fact that the one blade, which may be taken to represent 
the whole, moves in only one dire¢tion (it revolves round a 
given axis), it cuts out as it were the portion of water which 
corresponds with its area of revolution—a circumstance 
which greatly increases the slip, while it correspondingly 
diminishes the actual propelling power of the screw. It is 
otherwise with the screws formed by the tail of the fish and 
the wings of flying animals. ‘These are flexible and elastic, 
and, when they are made to vibrate, they are also made to 
reverse the direction of the stroke, and reciprocate in such 
a manner that the stroke from above downwards, or from 
right to left, as the case may be, is made to produce a 
current, which being met by the wing or tail when it makes 
a counter stroke from below upwards, or from left to right, 
greatly augments the propelling power. This holds true of 
every successive stroke made by the wing or tail. This 
power is further augmented by the elasticity and flexibility 
which contribute to the continued play of the natural screw, 
and by the fact that the wing of the bird and the tail of the 
fish strike at a great variety of angles—this peculiarity 
enabling them to diminish the slip to a minimum and to 
increase the propelling power to a maximum. Dr. Pettigrew 
* Nearly two years after Dr. Pettigrew wrote, Professor Marey, of Paris, 
obtained similar results by the aid of the sphygmograph; and since then M. 
Senecal, M. de Fastes, M. Ciotti, and others have been labouring in the same 
field. These investigators have confirmed Dr. Pettigrew’s original hypothesis, 
but, so far as we are aware, have added no new fa¢ts. 
