66 



NATURE 



[May 19, 1892 



The author states that M. Fergola, the astronomer at Naples, 

 may be looked upon as the one who first drew attention to this 

 question. Of the earlier astronomers, Sir George Airy was led 

 to the conclusion that the latitude was subject to a slight varia- 

 tion, and he published in 1854 and 1875 the greate-it and least 

 values for the co-latitude 38° 31' 22"-i6, and 38° 31' 2i"'35 

 respectively, obtained from observations of the pole. Many 

 other results were obtained by him, which caused him to assign 

 reasons for the fluctuations, but he deemed it wiser to publish 

 the results at a time when the measurement by graduated circles 

 was considered more concise. 



One of the first causes to which these variations were attri- 

 buted was refraction, and it was with the intention of settling 

 this point that Airy undertook with his zenith telescope the 

 measurements of the zenith distance of 7 Draconis, as this star 

 culminated near the zenith at Greenwich. M. Faye, towards 

 the year 1846, found out the advantages of such an instrument 

 as that used by Airy, and his installation was composed of three 

 instruments, a zenith telescope, a mercury trough, and a nadir 

 telescope, the last two of which provided a means of obtaining 

 the true nadir point. 



Porro, an Italian officer, adopted several of these improve- 

 ments in his instrument : he added to his telescope a trough 

 with a glass bottom, the plane surface of which was placed in a 

 horizontal position, and reflected feebly the image of the central 

 thread of the zenith telescope. By filling the trough with water, 

 another image of the same wire was obtained, which remained 

 visible during the transit of the star, and it was possible to take 

 several measures of the distance between the star and image. 



The next observer we find occupied in this research was 

 Respighi, who, in the year 1872, published the nadir distances 

 of several stars measured at Rome. The stars he observed were 

 those which culminated so near the zenith that they could be 

 seen in the telescope after reflection from mercury. From 

 a series of seventy-seven observations, taken during five months 

 of the year 1869, he observed the transits of two stars reflected 

 at his nadir. During this interval he found a difference of 2" '07 

 between the greatest and least of his results. 



In the method of Horrebow, the divided arc on his instrument 

 gave a rough reading of the inclination of his telescope, while 

 for greater precision he used the readings taken from a level 

 fitted to the telescope. 



M. d'Abbadie here condemns the use of levels altogether for 

 really accurate work, and backs his opinion with facts which he 

 has obtained from personal experience. He mentions that, as 

 far back as 1837, he made a study of their accuracy, but the 

 levels he used were not good ones. Later, after having pur- 

 chased some from the best-known makers in Paris, Munich, 

 London, and Hamburg, he repeated his experiments in a cellar 

 in an old chdteau, and he found that the results given were of a 

 most unsatisfactory kind. 



Admitting, then, that there was a variation in the latitude, it 

 was not long before periods were established. Peters, in the 

 year 1845, from observations at Pulkova, derived one of 303"9 

 days with a maximum on November 16, 1842. Mr. Nyren ex- 

 tended this to 305-6 days, with a maximum on December 13, 

 1867, while Mr. Downing, from ten years of observations made at 

 Greenwich, deduced a period of 306 'o days, with a maximum on 

 October 12, 1872. Leverrier, and Hough at Albany, also found 

 variations that were confirmed at Abbadia. 



M. d'Abbadie then refers to the variation of the true azimuth, 

 which, as he says, did not escape the notice of Airy. In the 

 year 1848 he estimated it as 4" or 5", while fifteen years later he 

 extended it to 6'' or 7". Of course, if the pole suffers any 

 displacement, such as an increase in elevation, at its two 

 elongations it will be displaced by the same amount, and the 

 azimuths in these cases would be increased. The greatest dis- 

 placement we have mentioned is 2" '07, but M. d'Abbadie says 

 " that if, by hypothesis, the north pole of the earth be elevated 

 by 7" by approaching the actual zenith, the true azimuth will be 

 diminished by those 7" in a place situated at 6h. om. of west 

 longitude, and increased by the same quantity at 6h. om. of east 

 longitude." He then states how, if the pole was considered 

 movable for places situated at opposite ends of a diameter of 

 the earth, the values for the variation should be the same, but of 

 opposite signs. To establish coincidences of this kind, it is sug- 

 gested that observers in Asia and America should take their 

 nadir readings at the same time as they are taken at Abbadia 

 — that is, in the morning and evening at 6h. Paris mean time. 

 The results Chandler obtained from his latitude observations 



NO. II 77, VOL. 46] 



indicated a minimum on September i, 1884, and a maxi- 

 mum on May i, 1885, with a difference of about o" 7. l?y 

 taking the 6 o'clock p.m. P.M.T. observations made at Abbadia, 

 it was found that a maximum value was obtained on September 

 I, 1884, and a minimum on May i, 1885, with a difference of 

 o""74. Contemporary observations made at Berlin and Hono- 

 lulu tended also to the same conclusion ; but in spite of them 

 M. d'Abbadie does not think it prudent to suppose a fluctuation 

 of the earth's axis. 



After referring to some sudden changes that this variation has 

 undergone, he goes on to mention Darwin's, Wolf's, and Pasch- 

 witz's instruments that were constructed for the measurement of 

 very small displacements. The last-named modified to a large 

 extent Zollner's horizontal balance, and added to it a mirror, 

 obtaining in this way, by the employment of photography, a 

 continuous series of curves. 



Mr. Nobile, in his memoir of 1883, related that, in 1820, 

 Brioschi believed in the small changes in the terrestrial lati- 

 tudes, and admitted two possible variations, one secular and 

 another periodic. He states, also, that Fergola, in 1871, sup- 

 ported this idea of Brioschi ; and Peters, as well as Nyren and 

 Gylden, confirmed this opinion. Euler and Legendre are also 

 said to have concluded from theory such a variation, giving it a 

 period of ten months. 



Another memoir by M, Nobile, contains a discussion on the 

 observations that were made with the object of determining the 

 latitude of the Observatory at Capo di Monte, near Naples, and 

 from these, together with some others, he deduced a tendency 

 in the latitude to increase in the summer and decrease in the 

 winter months. 



It will be seen from the preceding summary that very little is 

 definitely known as to the causes of this variation. From the 

 observations just referred to, it seems that refraction would be 

 the cause of such a variation, but as this is not borne out in other 

 observations, new theories must be advanced. M. d'Abbadie, 

 knowing the importance that is attached to the inquiries into the 

 causes of these variations, before concluding his letter adds a 

 few suggestions relative to a means of settling some of these 

 points, and the following is the plan which he proposes should 

 be adopted. 



Three observers, A, B, and C, should be provided each with 

 a good zenith telescope ; and the same two stars, which it is 

 proposed to use, should be observed by them. B and C should 

 be as near as possible on the same parallel of latitude, so as to 

 have identical refractions when measuring the declination on the 

 meridian of the chosen stars. To insure greater accuracy in 

 these declinations, he suggests that these stars should be observed 

 at their elongations with a geodetical circle, the refraction in 

 azimuth being zero, save in a few rare cases of lateral refraction. 

 The three observers should "notify at once, in a continuous 

 way, if possible, the varying movements of the nadir, and, in 

 every case, these variations at the precise moment where A 

 would observe on the meridian." 



To further complete this plan, two other observers, at D and 

 and E, might be added, the former situated at 6h. east, and the 

 latter at 6h. west longitude, in the same relative positions as Paris 

 is to Calcutta and Chicago. The position of E or D could be 

 chosen in the austral hemisphere, in order to determine whether 

 the variation of the nadir agrees with that which should be 

 observed simultaneously in the opposite hemisphere. Still 

 greater advantage would be gained if two other observers, situ- 

 ated at opposite points of the earth, were chosen to observe 

 these phenomena at the same instant. By adopting this plan, 

 a definite control would be had over the hypothesis that the 

 fluctuation was due to the movement of the terrestrial axis, and 

 if only this point could be settled, we should have advanced a 

 considerable step in its solution. 



We may mention here that quite recently Chandler has 

 made the remarkable discovery that the earth's axis of rotation 

 revolves round that of her maximum moment of inertia in a 

 period of 427 days. This, as Prof. Newcomb says, seems at 

 first to be quite contrary to the principles of dynamics, but, after 

 having investigated the theory, he finds that it is in perfect 

 harmony with the amount that the latitude varies, taking into 

 account the elasticity of the earth itself and the mobility of the 

 ocean. Radau's investigations were based on a 306-day period, 

 but he showed that the observed discordances would have to be 

 multiplied three times before they agreed with those obtained 

 by theory. 



W. J. L. 



