PROGKESSION IN OR THROUGH THE AIR. 121 



fig. 56. Thus if a & of fig. 56 be made to represent the 

 rod hinged at it travels through the space d h f in. the 

 same time it travels through j k I ; and through j k I in the 

 same time it travels through g h i ; and through g hi in the 

 same time it travels through e a c, which is the area occupied 

 by the thorax of the insect. If, however, the part of the rod 

 h travels through the space dbf in the same time that the part 

 a travels through the space e a c, it follows of necessity that 

 the portion of the rod marked a moves very much slower 

 than that marked b. The muscles of the insect are applied 

 at the point a, as short levers (the point referred to correspond- 

 ing to the thorax of the insect), so that a comparatively slow 

 and limited movement at the root of the wing produces the 

 marvellous speed observed at the tip ; the tip and body of the 

 wing being those portions which occasion the blur or impres- 

 sion produced on the eye by the rapidly oscillating pinion (figs. 

 64, 65, and 66, p. 139), But for this mode of augmenting 

 the speed originally inaugurated by the muscular system, it is 

 difficult to comprehend how the wings could be driven at the 

 velocity attributed to them. The wing of the blow-fly is 

 said to make 300 strokes per second, Le. 18,000 per minute. 

 Now it appears to me that muscles to contract at the rate of 

 18,000 times in the minute would be exhausted in a very 

 few seconds, a state of matters which would render the con- 

 tinuous flight of insects impossible. (The heart contracts only 

 between sixty and seventy times in a minute.) I am, therefore, 

 disposed to believe that the number of contractions made by 

 the thoracic muscles of insects has been greatly overstated; 

 the high speed at which the wing is made to vibrate being 

 due less to the separate and sudden contractions of the muscles 

 at its root than to the fact that the speed of the diff*erent 

 parts of the wing is increased in a direct ratio as the several 

 parts are removed from the driving point, as already ex- 

 plained. Speed is certainly a matter of great importance 

 in wing movements, as the elevating and propelling power of 

 the pinion depends to a great extent upon the rapidity with 

 which it is urged. Speed, however, may be produced in two 

 ways — either by a series of separate and opposite movements, 

 such as is witnessed in the action of a piston, or by a series 



