Aldis. — To describe a Straight Line by Link-work. 441 



concise order to go egg-hunting and not waste his time that 

 way." 



I do not propose to treat this subject from a scientific 

 point of view ; but the bones and ovens I saw at Waingongoro 

 in 1866, and the evidence obtained by the Hon. Walter Man- 

 tell in 1848, at Awanioa, certainly afford proofs that the moa 

 lived down to very recent times. 



AiiT. LIX. — On the Mechanical Description of a Straight 

 Line by means of Link-iuork. 



By "W. Steadman Aldis. 



[Read before the Auckland Instihdc, 23nd October, 1888.] 



Those who are familiar with the early history of the steam- 

 engine will remember that in the first form, knowm as 

 Newcomen's, the pressure on the piston was only employed to 

 pull the beam down, and that thus an attachment of the 

 piston to the beam by means of a chain passing over a circular 

 head was sufficient to insure the proper motion. When, 

 however, Watt closed in the cylinder and drove the piston 

 both up and down by steam-pressure, it became necessary to 

 connect the rectilinear motion of the piston-rod with the 

 circular motion of the end of the beam in a manner which 

 should enable the piston-rod to exert a push as well as a jsull 

 on the beam. 



The geometrical problem was to discover a means of 

 making one point move in a straight line while connected by 

 rigid bars with another point moving in a circle. 



The solution adopted by Watt was an approximate one, 

 and depended on the following geometrical proposition : — 



Let AB and CD be two fixed rods capable of turning in 

 the same plane about the points A and C, while their other 

 ends are connected by a bar, which is hinged to them at B 

 and D respectively. Then, if this arrangement of bars be 



