II. CURVE TRACING. 15 



7 la. Instrument for tracing with accuracy ellipses and spirals 

 up to 25 centimetres. M. Adrian Gavard, Paris. 



72. Instrument for drawing Conic Sections. 



Edward Uhlenhuth, Anclam, Pommerania. 



This instrument, which was invented by the exhibitor, shows in the first 

 place the formation of the parabola. By altering the arrangement according 

 to the instructions given in the sketches on the drawing board, the construction 

 of the ellipse and hyperbola easily follows. 



73. Elliptic Compass. jRenaud-Tachet, Paris. 



74. Colonel Feauceliier's Compound Compass. 



Conservatoire des Arts et Metiers, Paris* 



75. Compound Geometric Chuck, producing the kine- 

 matic retrogressive parabola, by continuous motion ; either on a 

 moving plane by a fixed point, or on a fixed plane by a moving 

 point. Henry Perigal, F.R.A.S. 



76. Machine for Compounding two Simple Har- 

 monic Curves. 



Invented and constructed by A. E. Donkin, M.A., Rugby 

 School. 



A strip of paper is wound round the cylinder ; the little glass pen moving 

 backwards and forwards on it draws one curve, a similar motion of the 

 cylinder the other. Since both move at once the curves are combined, and 

 the result rendered visible to the eye by the revolution of the cylinder. 



A. Eccentric for giving simple harmonic motion to pen. 



B. cylinder C. 



D. "I wheels for determining relative numbers of vibrations of pen and 



E. J cylinder. 



F. Wheel for transmitting slow motion to pinion Gr which turns the 

 cylinder. 



H. Idle wheel. 



I. Change wheels to supply different ratios of vibration of pen and 

 cylinder. 



76e. Spherical Rules and Squares for Spherical 

 Drawing. Dumoulin Froment, Paris. 



76a. Working Model of a Trace-Computer, designed by 

 the Exhibitor, for the use of the Meteorological Office. 



Francis Galton, F.R.S. 



Given two ordinates having the same abscissa, the instrument, of which this 

 is a working model, pricks out a third ordinate that shall be some desired 

 function of the other two. The original instrument was contrived for the use 

 of the Meteorological Office, where it is employed to derive a trace for humidity 

 from the traces of the dry and wet bulb thermometers. It consists of a hori- 



