August 24, 19 16] 



NATURE 



apparent zenith moves north and south about its mean 

 value, and that it is to this cause we owe the greater 

 part, if not the whole, of the Kimura term. Such a 

 displacement of the isobars is highly probable, and the 

 phase times of the latitude variation — nil at equinoxes, 

 maximum northward at summer solstice, and maxi- 

 mum southward at winter solstice — fits in perfectly 

 with this explanation. The observations made in the 

 southern hemisphere should form a crucial test. If 

 this is the true cause the apparent latitude of a 

 southern observatory will be shifted in the same direc- 

 tion as that of its northern counterpart, i.e. north- 

 ward in June and southward in December. We have 

 only a short series of observations from southern 

 stations, but so far as they go they appear to conform. 

 There is thus fairly strong evidence in favour of this 

 explanation. 



It must not, however, be assumed that the matter- 

 is settled beyond dispute. More observations are 

 necessary, and especially observations at widely 

 different latitudes. The international stations are, 

 as to the northern ones, almost exactly on a parallel, 

 and, as to the southern ones, on a parallel differing 

 only by 7^° from the northern. This uniformity, 

 highly advantageous for securing a precise record of 

 the motion of the earth's pole, is disadvantageous 

 for solving the riddle of the Kimura variation, and 

 other places should join in the attack. Unfortunately 

 the observations are ven,^ laborious and require the 

 almost exclusive attention of an observer. There is, 

 therefore, a very real want of an instrument which 

 shall demand something short of the whole time of a 

 skilled astronomer. With this object, and also with 

 the intention of eliminating certain sources of error, 

 instruments of new form have been devised. A short 

 account of these will be of interest. 



I shall not here attempt anv description of the 

 methods of observation used. It will be sufficient to 

 say that, as what we want to find is the direction of 

 the zenith at the place, all methods ultimately depend 

 either upon a level, giving us the horizontal plane, or 

 upon a plumb-line, giving us the vertical, and that of 

 these two the level is the one that has almost exclu- 

 sively been employed by the astronomer. The level is 

 an instrument capable of a high degree of precision, 

 but it has the disadvantage of being ver}- susceptible 

 to temperature changes, and, as both the glass tube of 

 the level and the spirit with which it is filled are bad 

 conductors of heat, it is impossible to ensure that it is 

 at an even temperature throughout. Irregularities 

 are thus produted which the reading of both ends of 

 the bubble only partially eliminates. The mere fact 

 of an observer standing near a sensitive level to read 

 it may seriously vitiate its accuracy. 



Some of these errors mav be avoided, and such errors 

 as are due to faulty reading of the level graduations 

 by the observer entirelv eliminated, by making the 

 level an integral part of the instrument by floating 

 the whole in liquid. The first application of this prin- 

 ciple to an astronomical instrument was bv^Chandler, 

 who carried out his series of latitude obser\'ations, 

 already mentioned, with an almucantar, being a 

 transit telescope floated in a trough of mercur>'. The 

 name " almucantar " means a small circle of the heavens 

 parallel to the horizon, and it will be sufficiently 

 obvious that if the telescope can be set at any angle 

 with the float, then as the instrument is rotated in 

 the trough, or the whole trough itself is turned, the 

 line of sieht of the telescope will move round such a 

 circle. With this instrument the stars are observed, 

 not as in a transit circle crossing a vertical line, but 

 crossing a horizontal circle of constant altitude. For 

 convenience of calculation this horizontal circle would 

 generallv be selected as that through the celestial pole 



NO. 2443, VOL. 97] 



at the place. Chandler's instrument was purposely 

 designed so as to differ as little as possible from the 

 ordinary visual type, and must have been a most 

 difficult instrument to use. The fact that he got such 

 excellent results from it is no small tribute to his 

 manipulative skill. The use of this form of instru- 

 ment cannot be said to have found great favour 

 among astronomers; there is only one example of it 

 in this country, and, so far as I'know, none on the 

 Continent. The one we have is at the Durham Uni- 

 versity Observator>-, and was designed by the present 

 Astronomer Royal for Scotland, in co-operation with 

 the late Dr. Common. It marked a very decided ad- 

 vance upon the earlier type. In two points special!^', 

 the screen of the floating part from wind disturbance, 

 and the attachment of the eyepiece to the fixed part, 

 the designers had the idea of a movable instrument, 

 which a slight touch or a puff of wind would set 

 vibrating to such an extent that no observation would 

 be possible for a minute or two, clearly before them. 

 The almucantar method of observation, meaning by 

 this, not the use of a floating type of instrument, but 

 the observing of stars crossing a horizontal circle, 

 though appropriate for the particular class of obser\'a- 

 tion we are here concerned with, those for determina- 

 tion of latitude, is not absolutely the best that can be 

 used. To reduce every possible source of error to a 

 minimum, particularly those due to refraction of the 

 atmosphere, we want to observe stars as near the 

 zenith as possible. 



The floating principle has been applied with great 

 success to a zenith instrument in the Cookson floating 

 zenith telescope now at Greenwich, designed bv the 

 late Bryan Cookson, whose early death was a great 

 loss to astronomy. 



It is a photographic instrument, with a telescope or 

 camera tube attached to a circular float w-hich floats 

 in a ring-shaped trough of mercury. The angle be- 

 tween telescope and float can be altered so that it 

 can be clamped to point either vertically upwards or at 

 any angle, up to about 30°, from the vertical. It is 

 used in the well-known Talcott method. A pair of 

 stars is selected which cross the meridian within a 

 few minutes of each other at nearlv the same zenith 

 distance, one north and one south of the zenith. The 

 instrument is set so as to include the first star in the 

 field, the lens is opened, and as the image of the star 

 moves across the plate it traces a fine line or trail. 

 .After the star has crossed the meridian, the telescope 

 is turned through 180°, leaving tube and float clamped 

 in the same relative position, and the second star 

 traces out its trail. The distance between the two 

 trails on the plate, w^hich is small if the difference of 

 their zenith distances is small, when the appropriate 

 corrections are applied, gives the observed difference 

 of zenith distance of the two stars, and, therefore, the 

 observed position of the zenith, and hence the latitude 

 of the obser\-er. By repeating the observation with a 

 number of pairs of stars a very precise determination 

 of the latitude is made. 



Recently a zenith telescope, designed, not on the 

 floating, but on the hanging principle, finding the 

 vertical line by virtue of its free suspension in a gimbal 

 ring, has been constructed, and would have been at 

 work by now had it not been for the interruption 

 caused by the war. Though it has thus not vet been 

 tested by practical experience, a few words on it may 

 not be out of place. The method of observation will 

 be the same as I have just described, except that there 

 is no arrangement for clamping the instrument at an 

 inclination to the vertical ; it is intended to be used 

 only in the vertical position, and the angle covered 

 bv the photographic plate will be a few degrees from 

 the zenith on each si^*». Exactly how far we can go 



