January 26, 1905] 



NATURE 



295 



if any, is supplied directly by actinium itself. From the 

 point of view of the theory of radio-active changes, this 

 shows that the change of actinium into AcX is a " ray- 

 less " change. 



A more complete account of these investigations will be 

 published later. T. Godlewski. 



McGill University, Montreal, January 2. 



A Simple Model for Illustrating 'Wave-motion. 



Mach's model for illustrating the transversal as well 

 as the longitudinal wave is known to work in a beautiful 

 manner. The arrangement for exciting the wave-motion 

 is not, however, very simple. The fact that the period 

 of a pendulum varies with the length of the string may 

 conveniently be availed of for producing a wave-motion in 

 a row of pendulum-bobs. 



As shown in the annexed figure, a series of pendulums 

 of equal length is suspended at equal intervals. Each 

 ball hangs on two strings, each of which passes through 

 the corresponding one in the row of holes in one of two 

 parallel horizontal rods m and n ; the strings pass through 

 the holes from inside to outside, and are tied together to 

 a horizontal rod l placed symmetrically above the two 

 rods. One end of the upper rod is pivoted, while the 



other can be raised to a suitable height. If this end be 

 raised, the length of the pendulums increases from the 

 end toward the other. 



The two rods, M and N, can be separated or brought 

 in contact by two links p and Q (not shown in the 

 figure), attached to their ends. If the rods be in contact, 

 the pendulums oscillate at right angles to the vertical 

 plane containing the rod L ; if they are separated, the 

 pendulums oscillate in this plane. Hence, by the position 

 of the links, the longitudinal as well as transversal oscilla- 

 tions of the pendulums can be excited at will. 



To produce a wave-motion, the end of the upper rod L 

 is raised, and then the two rods M and s are brought in 

 contact. Then the pendulums are set in motion simul- 

 taneously by a long rod. After one or two minutes the 

 phase-difference in each pendulum gradually increases, and 

 a beautiful transversal wave-motion is produced. The 

 wave-length becomes shorter and shorter ; if a wave of a 

 required wave-length is obtained, the rod l is lowered to 

 its initial position. Each pendulum has then an equal 

 length, so that wave-motion of a definite form incessantly 

 proceeds from one end to another. 



If the links be rotated, so as to separate the two bars 

 M and N from each other, the plane of oscillation of each 

 pendulum gradually changes, until the oscillation becomes 



NO. 1839, VOL. 71] 



at last longitudinal. Then a regular longitudinal wave is 

 observed to proceed from one end to another. 



On the other hand, a longitudinal wave can first be 

 e.xcited, and then be transformed into a transversal one. 

 Raising the end of the upper rod, and separating the two 

 horizontal rods M and N, each pendulum is simultaneously 

 set in a longitudinal motion by a long rod with receiving 

 holes for pendulum-bobs. A longitudinal wave is gradually 

 formed ; if a wave of a suitable length be obtained, the rod 

 L is lowered to its initial position ; then wave-motion of 

 a definite form is established. By turning the links the 

 longitudinal wave is transformed into a transversal one. 



Tokyo, Japan. K. Honda. 



Recently Observed Satellites. 



May I ask whether the small, distant, eccentric, and 

 possibly retrograde satellites of Jupiter and Saturn, which- 

 have been discovered and seem likely to be discovered, 

 ought not more properly to be regarded as cometary bodies, 

 or a shoal of meteors not yet too much drawn out for 

 visibility at a distance? Would it not be possible for the 

 larger planets to be attended by such bodies, the orbits of 

 which have been made moderately elliptical by an accidental 

 perturbation? It is known that the larger planets are able 

 to capture comets for the sun ; is it possible that with the 

 aid of their satellites and subsequent tidal action they may 

 be able to catch a few for themselves? 



Oliver Lodge. 



The University, Birmingham, January 20. 



Compulsory Greek at Cambridge. 



Mv e.xperience of Greek at Cambridge is very similar 

 to that of Mr. Willis, but the slight differences are, I 



think, instructive. 



When I decided to go up to Cambridge to study mathe- 

 matics and philosophy I was living abroad, and I crammed 

 Greek just as Mr. Willis describes, except that I worked 

 entirely alone. But on going in for the " Little Go," 

 though I passed easily in translation, I failed by a few 

 marks in Greek grammar. It was so near a thing that I 

 thought I might pull through in December with a few 

 hours more grind ; but unfortunately I ran it too fine, and 

 again failed by a few marks. This meant that I had to 

 get up a complete new set of translation books for the 

 following June, and to prevent further mistakes I went 

 to a coach for the grammar part. I then passed, getting 

 a second class. Like Mr. Willis, I can only say my 

 present knowledge of the language is ynl, although I had 

 a double dose of it. It cannot for a moment be pretended 

 that I got any insight into " Greek thought " which I 

 could not have got equally well by reading a good trans- 

 lation. But I confess my opinion of the value of Greek 

 thought was not raised by what I read — at best it only 

 seemed to me creditable, considering how long ago it 

 was written. But this may have been due to my resent- 

 ment at being forced to waste time in an uncongenial 

 study, when I was. keen to get on to something else. 



Edward T. Dixon. 



Racketts, Hythe, Hants, January 20. 



Super-cooled Rain Drops, 



Walking home from the university last night at about 

 8.45 p.m. an interesting phenomenon occurred. 



Something was falling which at first appealed to one. 

 as hail, but I soon found that it was large rain drops 

 evidently cooled below the freezing point ; at the moment 

 they struck objects such as one's hat, coat, or walking 

 stick, &c., they instantly solidified in small hemispherical 

 lump's ; falling on the ground they gave it the appearance 

 of a sheet of ice, but the roads were not slippery, as the 

 solidified rain gave the road just a nice amount of rough- 

 ness. The noise of the falling rain was very curious— a 

 crackling noise, not unlike that of small electric sparks. 

 Edward E. Robinson. 



The University, Birmingham, January 17. 



