SCIENCE-GOSSIP. 



245 



of this class ; and here I will venture to suggest that 

 the curious " rice-grain markings" observed on the 

 surface of the sun are simply intersection figures 



Fig. 6. — Twin-elliptic curves. 



resulting from the super-position of cloud and 

 vapour currents, which are themselves compound 

 vibration curves. I would 

 venture to further suggest 

 that in such considerations 

 will be found the order of 

 operations that determine 

 the markings and figura- 

 tions of the coats of 

 animals, insect wings, 

 flower-petals, etc. 



Here, two considerations 

 must be borne in mind. 

 First, vibration figures 

 may be produced without 

 vibration ; a circle with 

 a pair of compasses for 

 example ; second, a vibra- 

 tion is not necessarily a 

 quick movement ; it may 

 have any period whatever, its chief characteristic 

 being that it will be recurrent as long as the con- 

 ditions that give rise to it remain unaltered. 



Sound-curves are not essentially different to 

 vibration figures in general ; but musical curves 

 include the simplest possible combinations of 

 vibration. 



The twin-elliptic pendulum (which I exhibited 

 at the Royal Society's Soirees in April and May 

 last) is a simple and effective apparatus for the 

 production of such combinations of motion. By 

 this arrangement (see diagram) friction is reduced 

 to the lowest possible minimum, and there is 

 nothing to interfere with the absolute freedom of 

 natural motion beyond the structure and stability 

 of the suspension-point. 



The chief, or fundamental, suspension-point, on 

 which the whole system swings, is the extremity of 



Fig. 8. — Twin-elliptic curves. 



a i-inch sphere, made of the very hardest steel, 

 firmly fixed in the point of a strong " ceiling- 

 hook." 



The point in the "suspension-crank" bearing 

 directly upon the steel sphere is a polished cavity 

 in a steel slab, also extremely hard. From this 

 point of contact there is a motion of the whole 

 structure, always in some phase of the ellipse. 

 Fixed firmly to the under side of the carrying- 

 board is a strong brass plate, from the centre of 

 which hangs a stout silk thread carrying the "de- 

 flection weight." This weight also moves in an 

 ellipse, considered in reference to its own sus- 

 pension-point. These two elliptic motions are 

 simultaneously communicated and received by the 

 two pendulum-masses. The consequence is that 

 in reference to the pen-point, which is a fixed point — 

 outside the pendulum system —the visible resultant 

 is a combination of the two ellipses. 



Every movement of the pendulum is therefore a 

 twin-elliptic curve, and every such curve represents 

 (it is verily the vibration-path of) a musical interval, 

 whose vibration ratio is identical with the com- 

 parative rates of vibration 

 of the two ellipses, which 

 is the same thing as the 

 ratio of the elliptic periods ; 

 or, in common words, the 

 rate at which the whole 

 pendulum goes round, in 

 comparison with the rate 

 at which the deflector goes 

 round. 



The possible ratios for 

 an ordinary pendulum 

 (from eight to fourteen feet 

 in length) are from about 

 3:2 to 4 : 1. Between these 

 extremes there are about 

 200 available distinct ra- 

 tios, and for every ratio 

 of phases and modifications 



an endless variety 

 of amplitude. 



Fig. 7.— Twin-elliptic curves. 

 Beyond this inexhaustible variety of initial forms 

 or outlines, every figure admits of a wide range oi 



