176 Transactions. — Miscellaneons. 



oblique stroke. As the anterior edge of the wing is only slightly elevated in 

 flight, a considerable upward pressure is received during the down-stroke of 

 the wing as the air reacts on the concave side. As this stroke mvolves the 

 most work, therefore the muscles which effect it are of great size and 

 strength. We must not suppose that the up-stroke of the wing is only 

 useful to so raise it as that it can be again depressed, for in the up-stroke 

 we may detect one of the main provisions for its onward transit ; for in the 

 up-stroke, as the anterior portion of the wing is still more or less elevated, 

 it is clear that a powerful forward impulse is thus given to the wing and 

 thence to the bird. 



As it is much easier to move the wing upwards than downwards, as the 

 upper and convex side meets less resistance from the air, therefore the 

 upward stroke is effected by smaUer and weaker muscles, acting probably in 

 a more disadvantageous position. 



From the form and structure of bh-ds and from the foregoing observa- 

 tions, it is apparent that a comparatively small force will start and propel 

 them horizontally to that which is requh'ed to sustain them against the 

 force of gravity ; hence the attempt in the model before us to so construct the 

 sails or wings that the tips are kept at a small but precise angle, and the 

 whole length thereof is by its construction, when in action, compelled to 

 assume a slight concavity. There is, however, a limit to this concavity, for 

 as the principal part of the weight of the wheel is at the circumference, 

 therefore the greater the speed the greater is the centrifugal force of all parts 

 of the cu'cumference, thus increasing the tension; the very slight elasticity 

 of the rim thus coming into action. 



Aet. XVI. — Introduction of the Principle of the Gyroscope in Aerial Transit. 



By Henby Skey. 

 Plate IV. 

 [Read before the Otago Institute, 9th October, 1877.] 

 All our ideas of stabihty of direction and position are but relative, and the 

 same law which expresses that all matter moves in straight lines until 

 deflected by another force, and which regulates a planet in its path, can 

 give stability to an aerial machine, so that it can be propelled and steered 

 in any direction even against moderate winds ; indeed the most permanent 

 thing in nature is the axis round which a symmetrical body rotates ; we 

 have many familiar instances of the permanency of the plane of rotation, 

 from the child's missiles, tops and hoops, then again the wheels of bicycles, 



