Sept. 3, 1874] 



NA TURE 



359 



tion of the works of Foucault, and at the expense of the 

 Imperial treasury. It was presented to the Academy of 

 Sciences on December 13, 1869, then given by Napoleon 

 III. to the Observatory, where it has been installed since 

 1S73. _ . 



The instrument, as designed by Foucault, of which M. 

 Wolf has published a complete and detailed account, 

 rests on a brass stand supported by three screws, with 

 two levels placed crossways, and a regulating azimuth 

 movement. There are three distinct parts — the mirror 

 and its mounting, the polar axis and the mechanism which 

 connects this axis with the mirror, and lastly the regulator. 

 The plane mirror, 30 centimetres in diameter, was con- 

 stnicted by M. Ad. Martin, according to the method de- 

 vised by Foucault ; it is carried by a horizontal axis on 

 the top of two vertical supports, which turn round a 

 centre. This movement is perfectly effected by means of 

 a circle of small wheels placed at the foot of the supports. 

 The mirror is kept in its mounting by means of cleats 

 and spiral springs, in order to avoid all irregularity of 

 surface. In the centre of the mounting is fixed perpen- 

 dicularly a directing handle, which slides through a ring 

 carried by a fork jointed to the lower extremity of the 

 horary axis. The direction of the incident ray being 

 that of the axis of the fork, and the length of this fork 

 being equal to the distance of its point of articulation 

 from the horizontal axis of the mirror, the line which 

 measures that distance gives the constant direction of the 

 rellected ray. 



Finally, a clockwork movement, the isochronous regu- 

 lator of Foucault (Fig. 2), placed at the foot of the instru- 

 ment, communicates to the mirror a motion sensibly equal 

 to the diurnal motion, so that the celestial bodies main- 

 tain iavariable positions in the field of a horizontal tele- 

 scope, in front of the apparatus directed towards the 

 mirror. 



The entire apparatus, the principle of which is the 

 same as that of the heliostat, rests on a triangular support ; 

 a hole on the north sidereceivcs the weight which drives the 

 clock. A wooden cabin, moving on wheels from north to 

 south, forms a shelter for the instrument. For the purpose 

 of observation the siderostat is completely exposed by 

 rolling the hut towards the north. The telescope, sup- 

 ported on two pillars, is placed in a brick hut, some little 

 distance from the siderostat ; this hut is very slightly 

 elevated for the purpose of intercepting the least possible 

 portion of the southern sky. A telescope with a mirror 

 of silvered glass, pierced in the centre to receive the eye- 

 glass, is the one at present employed. 



If it is desired to bring into the telescope the light pro- 

 ceeding from a star whose polar distance and right ascen- 

 sion are known, this is done by two circle;, which corre- 

 spond, the one to the polar distance and the other to the 

 horary angle for the moment of observation in the usual 

 way. Then, the circles being fixed, the clockwork is put 

 in motion and the mirror throws continuously into the 

 telescope the rays proceeding from the star under observa- 

 tion. The clock movement, already applied to some great 

 equatorials, is perfectly regular, and obtained for its clever 

 maker, M. Eichens, the grand prize in the mechanical arts 

 at the Universal Exhibition of 1S67. 



It was necessary to possess, for the siderostat, some 

 means of adjustment so as to be able to vary in very 

 small quantities the horary angle or the polar distance 

 vithout stopping the movement. The former variation 

 is obtained by means of a subsidiary wheelwork which 

 has already been long in use. l)ut the variation of the 

 polar distance was more difficult to accomplish ; M. 

 Xichens, however, has solved the difficulty after a very 

 iigenious fashion. 



The siderostat, since its construction, has been almost 

 exclusively employed forphotographic experiments in con- 

 nection with the approaching transit of Venus. Con- 

 secuently we do not yet know what results we have a 



right to look for. But in the ideal of Foucault, the 

 instrument ought to be an indispensable auxiliary of 

 physical astronomy ; this is its proper purpose. Experi- 

 ments which demand perfect steadiness will be advan- 

 tageously made, such as the measure of the positions of 

 spectrum lines and of the displacement of these hnes by 

 means of fixed spectroscopes of large dimensions. It is 

 easy to conceive, besides, the numerous advantages result- 

 ing from the fixed direction of reflected rays. We may 

 henceforth adapt, with the greatest ease, to the observing 

 telescope, the apparatus necessary for the work of celestial 

 photography for photometric researches. 



The complete instrument, telescope and siderostat, 

 placed in the plane of the meridian, may be regarded as 

 a meridian instrument ; and the determination of the 

 right ascensions and polar distances of known stars will 

 enable us to rectify the adjustment already made of the 

 relation between the telescope and the siderostat. The 

 purpose is evidently thus not to obtain a transit instru- 

 ment, but only to get an approximation equal to that of 

 equatorial observations. It is, besides, always in our 

 power to increase the precision by comparing the star 

 under observation with a well-known neighbouring star. 



Observations by means of the siderostat may be made 

 in two ways — with the mirror fixed, or turning under the 

 action of the clockwork. In the former case, the instru- 

 ment becomes to some extent an equatorial, but with the 

 advantage to the observer that he has not to change his 

 position. An inconvenience appears here ; each time that 

 the mirror is moved the direction of the apparent move- 

 ment changes, and consequently it becomes necessary 

 to make a new adjustment of the micrometer threads. 



This inconvenience is more serious if, when the mirror 

 is in motion, it is desired to effect measurements of 

 double stars. In this case the direction of the diurnal 

 motion changes the angles of position. It is then neces- 

 sary to measure the angles of position by starting with 

 the vertical and the horizontal, and, by means of the hour 

 of obsei-vation, reducing them to the ordinary form. 



The real defect of the siderostat, which, however, it 

 has in common with all other instruments of observa- 

 tion, is that it does not enable us to examine the entire 

 heavens. But the most interesting region for research is 

 comprised between the pole and the southern horizon, 

 and the siderostat which we have described permits ob- 

 servations between these limits. Should it be desired to 

 investigate the rest of the sky, a second siderostat would 

 be necessary, reflecting the rays towards the north. 



Let us not, in conclusion, forgetthat the reflection from 

 the mirror of the instrument causes a slight loss of light ; 

 the propartion of light reflected is constant and equal to 

 93-100 of the incident light for new silver. 



From this description it is clear that it is only from the 

 standpoint of physical astronomy that the employment of 

 the instrument will be most useful ; and no doubt, in this 

 direction, it will give numerous and important I'esults. The 

 problems of the universe offer, indeed, a productive and 

 inexhaustible mine, and the new astronomy, with its 

 powerful means of investigation, gives us reason to hope 

 that future researches will bring to light some brilliant 

 discoveries. 



NOTES 



The WcsiciH Morniii^ A'eifs has received from its correspondent 

 on board the Challenger xa account of the voyage to New Zealand, 

 which has been stormy and protracted. The result of the sound- 

 ings has been most satisfactory, and it is confidently expected 

 that New Zealand will be telegraphically connected with Eu- 

 rope next summer. The bottom was sand and mud, gradually 

 shelving to a depth of 2.600 fathoms, at which it remained very 

 evenly for a long distance, the temperature at this depth being 

 33 degrees and at the surface 64 degrees. At this point the 



