402 REPORT— 1892. 



and certain other elliptic compasses, and the conicograph of Dr. Zmurko, 

 of Lemberg, which describes all three conic sections. 



Since the above date there have been several improvements in instru- 

 ments for drawing ellipses, amongst them the conograph, which is the 

 invention of Mr. A. 0. Campbell, of Bridgeport, Conn., U.S.A., described 

 in the book of Professor MacLay abovementioned, an account of which 

 is contributed to the ' Transactions of the American Society of Mechanical 

 Engineers,' vol. viii. p. 145. 



The most important of recent contributions on the subject of ellipto- 

 graphs is that by Professor Alexander and Mr, A. W. Thomson.' The 

 authors mention three modes of generating the ellipse — by (1) a point 

 fixed to the body with two or more studs, and oscillating in straight 

 Blots ; (2) a point rigidly fixed to a circular wheel, rolling inside another 

 of double its diameter ; (3) a point describing an antisatellite motion of 

 double angular velocity round another describing a circular orbit. 



The first is, of course, the old elliptic trammel ; but they have designed 

 an important improvement, to obviate the objection in the ordinary 

 arrangement of the impossibility of obtaining with the ordinary pen a 

 clean line of uniform thickness, since the pen gets broadside at certain 

 parts of the curve. Their arrangement consists of a steering-needle, 

 which always forms the normal to the ellipse being generated, the 

 principle of a circle rolling in another twice its radius being employed to 

 attain this result. 



The second improvement they mention Is an example of the anti- 

 satellite method of describing an ellipse, which is used for teaching 

 purposes in connection with Rankine's ' Method of the Ellipse of Stress.' 

 This arrangement, which is used on a blackboard, consists of a T-square 

 pivoted to the board at the butt, and a pointer pivoted to the blade of the 

 T-square. At the back of the blade, where the pointer is pivoted, is a 

 wheel fixed, on this wheel an endless chain is wrapped, which passes 

 round a wheel fixed to the board at the point where the butt is pivoted ; 

 hence, if the blade of the T-square is turned to the right, the pointer 

 automatically turns to the left, so that the bisector of the angle between 

 the pointer of the blade is always vertical. By this arrangement the end 

 of the pointer describes an ellipse. 



They next describe an improved elliptograph, for continuously de- 

 scribing an ellipse of every eccentricity, by means of a wheel rolling in a 

 circular rack of twice its diameter. The circular rack may be dispensed 

 with if a gauged endless chain wrapping round two wheels is used. 

 Further modifications are also shown, one designed by Mr. Yeates, which 

 is a frame in the form of a ring standing on four legs, armed with prickers 

 to pierce the drawing-board so as to hold the frame steady, and at the 

 same time mark the position of the principal axis. The lower ring, 

 concentric with the other one, runs in four grooved wheels, one in each 

 leg. One difficulty in the construction of this mechanism is to make the 

 lower ring, which runs in the grooved wheels, truly concentric with the 

 cross formed by the slots in the principal ring. 



Yet another mechanism is described, in which bevel wheels and a 

 framework form a part, and a steering-needle being always on the radius 

 of what corresponds to the planet in the anti-satellite motion. This 



* ' Elliptographs,' by Thomas Alexander, M.A., and A. W. Thomson, D.Sc. 

 Transactions of the Royal Irish Academy, vol. ssix. p. 673. 



