98 



NATURE 



\Nov. 30, 1876 



may answer the purpose for others as it lias now for some time 

 answered mine. Thomas Fawcett 



Blencowe School, Cumberland, November 6 



Electric Motor Pendulum 



The following very simple apparatus may I think be of use in 

 any laboratory or other place where at times it is necessary to 

 have a pendulum beating seconds in order to give the time for 

 any experiments needing it. 



It consists of a Siemens' galvanoscope, A, to which is attached 

 the pendulum ; the needle N, preferably with platinum contacts, 

 works between two platinum wires, B and c, with a small amount 

 of play ; these platinum wires are insulated from one another by 

 being fastened into a piece of ebonite, which works on a pivot D. 

 The needle is connected by its support to one end of the coil of 

 the galvanoscope, the other end being to earth. To the wires, 

 B and c, are connected the opposite poles of a small battery, the 

 centre of the battery being to earth. 



The action of the instrument is as follows : — On slightly oscil- 

 lating the pendulum the needle N makes contact between B say, 

 and the coil, the magnet being so arranged that the needle then 

 deflects towards B, thus carrying with it the movaVjle contact 

 wires until the pendulum reaches its limit of oscillation, when it 



via 



-^ 



falls, breaks contact with B and makes contact with C, which 

 thus tends to pull the needle over to c, and so on ; in this way 

 the pendulum receives at each oscillation the impulse necessary 

 to overcome the forces tending to itop it ; and thus will keep 

 oscillating as long as the battery supplies the motive power. 

 For small arcs the beat is not affected by variation in battery 

 poTter. 



In the circuit of the battery we can introduce an electro- 

 magnet which at each contact of the pendulum on one side will 

 make a stroke on a bell, or indeed by a detent Vv-ill move by a 

 small train of wheels the hands of a clock. If the pendulum is 

 made to beat half seconds, then the contact being made alter- 

 nately on each side, the bell stroke would beat seconds. We 

 could of course introduce any number of arrangements of this 

 sort at any intervals along the circuit, and so move any number 

 of clocks at different positions in a larj^e establishment, only 

 one pendulum being requisite to control the whole set. 



P. HiGGS 



PROF. YOUNG ON THE SOLAR SPECTRUM 



n^HE paper of Prof. C. A. Young read at the last 

 -»■ meeting of the American Association for the 

 Advancement of Science describing his recent measures 

 of the displacement of the D and other absorption lines at 

 the receding and approaching limbs of the sun, has a 

 double interest. 



By careful measures to which all the necessary correc- 

 tions have been rigorously applied, obtained by using a 

 diffraction grating in combination with a prism, Prof. 

 Young deduces from observation of the D lines a value of 

 I '42 ± o'035 miles per second for the surface velocity 

 of rotation at the sun's equator Direct observation of 

 the motion of spots gives V2$ miles per second, and 

 the author thinks that the difference of these two 

 values being so many times larger than the probable 

 error of the spectroscopic method, the result of which 

 agrees so well with Vogel's result, indicates that a por- 

 tion of the displacement observed is produced by the 

 difference in the angular velocity of rotation of the solar 

 atmosphere which causes the absorption lines and the 

 underlying luminous surface, and the sign of the differ- 

 ence would indicate that the atmosphere is swept forward 

 with the greater velocity of the two. 



This conclusion is itself one of great interest, but for 

 many persons the fact that it is based on the acceptance 

 of Doppler's theory will be a source of satisfaction as 

 indicating that the recent disputes as to its soundness are 

 beginning to be considered settled and in its favour, as 

 at any rate a near approximation to the truth. 



One of its first assailants, on matheinatical grounds, 

 was Prof. Petzvall, But, as was pointed out by Mach in 

 a "Contribution to Doppler'sTheory," published at Prague, 

 in 1874, his main argument fell beside the mark, while the 

 only one which touched it went to prove that for com- 

 paratively small velocities of translation in the source of 

 sound or light, compared to the velocity of wave trans- 

 mission, Doppler's theory was a correct approximation. 



More recently Van der Willigen's mathematical objec- 

 tions have been apparently fairly disposed of by Mr. 

 Christie, while the discrepancy that Father Secchi has 

 lately pointed out between the measures of displacement 

 of spectral lines in the case of certain stars as observed 

 by Mr. Huggins on the one hand and at Greenwich on 

 the other, does not really affect Doppler's theory at all, 

 but only the degree of certainty with which it can be 

 applied to the determination of stellar motion. But the 

 facts are not as Father Secchi represents them. He points 

 out, in a list of thirteen stars, that the displacement in 

 the case of some five stars as observed by Huggins is in 

 the opposite direction to that observed at Greenwich. 

 But the Greenwich observations that he takes are some 

 early tentative observations. We have taken the trouble 

 to refer to the most recent Greenwich measures, and find 

 that of the five disagreements insisted on only one holds. 



INDIAN GEOLOGY 



I'^HERE seems to be a very pretty quarrel just now— 

 and one urged with the usual absence of acrimony 

 in scientific controversies — as to the age or ages of an 

 important group of rocks in Her Majesty's Indian 

 empire. 



For years it has been known that while a large mass of 

 the rocks forming the Peninsula of India are unfossi- 

 liferous, there is also in that country an extensive series 

 of beds the predominant, and frequently the only, fossils 

 of which are vegetable remains. These beds were often 

 spoken of as the Plant-beds of India. Among the flora 

 certain forms which used to be called PalceosamicF, novv 

 Ptilophyllum, were pretty generally distributed, while the 

 genera Schizoneura and Glossopteris were found in lower 

 portions of the series. 



On the evidence of the first-named fossils and several 

 others, a Jurassic age was assigned to the containing beds, 

 while the identity of the Glossopteris with Australian 

 forms involved these Indian beds in the dispute as to 

 whether the coal-rocks of that country were likewise 

 Jurassic or really carboniferous. 



One portion of the Indian plant beds contained a 

 limited terrestrial fauna which on high authority (Hux- 



