NOTES. 



ASTRONOMY. 



By A. C. D. Crommelin, D.Sc. 



THE CiKO-COMPASS. — This instrument was described 

 at the recent meetings of both the Royal Astronomical Society 

 and the British Astronomical Association. It may claim a 

 distinct astronomical interest from the close relationship that 

 has always existed between Astronomy and Navigation, and 

 also because it points to the astronomical instead of the 

 magnetic north, its driving force being entirely non-magnetic, 

 and arising from the earth's rotation. The benefit from the 

 navigational point of view is fairly obvious, for not only is the 

 magnetic deviation different for every point, but even at the 

 same point it undergoes a slow secular change, so that the values 

 marked on the charts need to be continually revised ; there is 

 the further obviation of all magnetic difficulties arising from 

 the iron in the ship. This trouble had been partially corrected 

 bv Airy by the use of compensation magnets, but there was 

 still a residual effect due to the change in the ship's magnetism 

 when she changed her course, and so on. The driving force in the 

 new compass is sufficiently powerful to enable a gearing-up 

 arrangement to be used, an inner card giving angular deviations 

 thirty-six times those of the outer one, or an entire revolution for 

 10', so that a change of course of a small fraction of a degree is 

 rendered manifest ; further, the readings of a single compass 

 (which in the case of warships can be placed in a secure 

 position below the water-line) can be electrically reproduced 

 on dials in various parts of the ship. There is a very ingenious 

 arrangement for damping the oscillations from true north by the 

 changing effect of an air jet escaping from the box containing the 

 gyroscope. A diagram was shown in which t'ne compass was 

 pointing some 50° from north w-hen the gyroscope was started. 

 It only took a few minutes to find the meridian within a few 

 degrees ; swings continually diminishing in amplitude took place 

 till after four hours they became insensible. The gyroscope 

 should be started a few hours before the commencement of a 

 vovage. It is turned by an electromotor ; hence a current of 

 electricity is required, but this is now available on all large 

 ships. 



It is of interest to compare this instrument with that devised 

 bv Foucault to demonstrate the earth's rotation. This was in 

 addition to his well-known pendulum experiment, which was 

 repeated at the Pantheon a few years ago. His gyroscope had 

 freedom in all three axes, and thus, as Proctor pointed out in 

 "Old and New .Astronomy" page 236, it behaved "like an 

 independent planetary body with its own proper polar axis 

 directed constantly to the same point of the celestial sphere." 

 The gyro-compass has not so much freedom, being suspended 

 pendulum-wise by means of a float in a mercury trough. The 

 idea of giving it complete freedom was considered, but 

 abandoned, as it was found to be almost impossible to obtain 

 such perfect balance about each axis that it could be depended 

 upon to keep its direction in space for any length of time. 

 Foucault's principle was practically used by Professor Piazzi 

 Smyth on his voyage to Tenerift'e, described in " Teneriffe, an 

 Astronomer's Experiment." .A telescope was mounted on 

 board ship attached to a Foucault gyroscope, and it was found 

 that celestial objects were kept for some time in the field, in 

 spite of the rotation of the earth and the rolling of the ship. 

 Unfortunately the flywheel was unequal to the strain, and gave 

 way before long, but sufficient was done to illustrate the 

 principle. 



THE APPLICATION OF PHOTOGRAPHY TO 

 FUNDAMENTAL ASTRONOMY.— Since the introduction 

 of astronomical photography, many minds have been at work 

 in the endeavour to apply the new' method to astronomy of 

 position. Of course the difficulty is that either the plates 

 cover only a very small region of the sky, or they are of 

 too small a scale to give accurate positions. Hitherto most 

 of the methods suggested to give results over more extended 



regions have made use of the existing Tr -..Uiit-Circle, simply 

 substituting photography for visual work at some of the 

 stages. Professor Turner proposed some new methods in 

 a paper read before the Royal .\stronomical Society in March, 

 which gave rise to an interesting discussion. His proposals do 

 not aim at the independent formation of an entirely new 

 catalogue of stars, but ( II to obtain dift'erential places of further 

 faint stars with the aid of the existing places of the brighter 

 stars in the same zone, and 12) to detect systematic errors 

 in the existing catalogues, such as the large error of a periodic 

 kind that formerly affected the Greenwich clock stars, and 

 which ajiparently has not even now been wholly removed. 

 For the first problem he suggests the use of a coelostat reflect- 

 ing a region of the sky into a fixed horizontal telescope, 

 .^t the same time a small beam of light from a fixed source is 

 made to fall on the coelostat and thence on the plate. This 

 beam can be occulted by the clock pendulum at each 

 swing, and thus serves as a time record. An exposure is 

 gi\en sufficient to record stars dow-n to the tenth magnitude or 

 so. After ten minutes (say) the coelostat is moved back to the 

 first position and the process repeated. This can be done as 

 often as desired till there are pictures of (say) twenty regions 

 on the plate, all of the same declination, and differing in 

 Right .A.scension by known amounts. There will in the end 

 be enough known stars on the plate to obtain good dift'erential 

 positions of all the stars on it. And it is claimed that the 

 labour and expense would be far smaller than if all were 

 observed with the transit-circle. 



For the second problem he suggests the use of two horizontal 

 telescopes at right angles, both facing a fixed mirror. Two 

 fiducial marks in the two telescopes, exactly 90" apart, are 

 found by simultaneous observations of the marks and their 

 reflections in the successive faces of a silvered cube, in the 

 place subsequently occupied by the mirror. Simultaneous 

 photographs of two regions in the sky 90' apart are then taken, 

 the following being done by moving the plates, not the mirror. 

 The correction for refraction is discussed, and shown to be 

 nearl}' constant if the altitude of the two regions is the same. 

 It is pointed out that for observatories in high latitudes the 

 most suitable study would be the observation of regions of the 

 same declination, 90 apart in Right Ascension, while for 

 equatorial ones, stars near the equator and poles might 

 usefully be compared, thus checking our system of fundamental 

 declinations. 



Professor Turner has obtained a Government grant for 

 prosecuting experiments of the kind here briefly indicated, and 

 the results will be awaited with interest. 



MARS. — Professor Lowell writes to me pointing out that he 

 had previously used the method of shift due to motion in the 

 line of sight, in the endeavour to separate the lines of Martian 

 and terrestrial water-vapour in the spectrum of Mars ividc 

 " Knowledge" for February). Like Professor Campbell, he 

 did not succeed in getting definite results from the method, and 

 found the comparison of the Martian spectrum with that of 

 the moon more suitable. 



A series of glass positives of this planet, taken by Professor 

 Barnard with the great Yerkes Refractor, were exhibited at the 

 R.A.S.. and are reproduced (necessarily with some loss of 

 detail.) in the Monthly Notices for March. These photographs 

 are on a larger scale than those taken at Flagstaff, and are, I 

 think, superior to these in the dehneation of the dusky regions 

 (the so-called Seas) of which the outlines are very sharply 

 shown, also the various depths of shading in different parts. 

 They do not, however, show as many canaliform markings as 

 the Flagstaft" ones, though a few are indubitably shown. The 

 extreme narrowness of these markings makes them very 

 liable to be affected by bad definition, and that at Yerkes is 

 said to be inferior to that at Flagstaff, which was carefully 

 selected for this very purpose, and is at an altitude of seven 

 thousand feet, surrounded by a desert. 



185 



