September 7, 1893] 



NA TURE 



455 



internal ci.nes {LionvilW s yottrml, 2i(l serie;, tom; iil. 1S58, 

 p. i), anl these modifiej forms had bjen employed by Darwin 

 in the di'scussion of the influence of geological changes in the 

 earth's axis of rotation. {Pkil. Tram. 1877, p. 271.) 



No suspicion, however, seems to have entered the brain of 

 any of these investigators that any modification of Euler's 305- 

 day period would result either from the mobility of the ocean, 

 or the elastic yielding of the earth as a whole. 



Newcomb sho'A-s in a very simple manner how this result 

 might follow {Mjiithly Notices K.A.S. March 1892, p. 336), 

 for in consequence of this elastic yielding the pole of figure 

 would be brought towards the pole of the instantaneous axis by 

 the centrifugal force. 



Let us call the undisturbed position of the pole of figure the 

 fixed pole, the actual position at any instant the movable pole, 

 and the pole of the instantaneous axis the pole of rotation. 

 The movable pole is therefore constantly moving towards the 

 pole of rotation, describing a sort of curve of pursuit ; the 

 instantaneous velocity of the latter about the former is that of 

 Euler's period, but the effect of the motion of this movable 

 pole is to diminish the velocity with respect to the fixed pole in 

 the ratio of its distance from the latter to the distance from the 

 pole of rotation. 



Lord Kelvin remarks that this supplies a new and indepen- 

 dent method of determining the effective rigidity of the earth. 

 As will readily appear, in this distortion work is being done 

 against resistance, and unless the earth be perfectly elastic, 

 which is certainly not true of that part accessible to observa- 

 tion, the two axes would in time be brought into practical 

 coincidence. The tidal action set up in the oceans uould also 

 tend to produce the same result. Apparently, then, the con- 

 tinued existence of this term requires a constantly recurring 

 series of impulses. 



Gylden remarks that the hypothesis of elasticity is not the 

 only one which will explain the Chandlerian period. (Astrono- 

 mische Nachrichten, Band 132, p. 193.) He also concludes as 

 the result of a mathematical analysis that we must look for the 

 impelling cause to cjncussions going on in the interior cavities 

 of the globe. 



Aside from the fact that these discussions are in need of ex- 

 planation to an extent quite equal with that of the phenomenon 

 itself, it is an open question whether any explanation is called 

 for. We have no proof of the perpetuity of this term. We 

 are in possession of no observations accurate enough to throw 

 any light on this subject before the time of BraHley, nor can it 

 be asserted that so small a coefficient has remained constant 

 during the interval of 150 years ; possibly it may be on the 

 road to extinction. 



As to the annual term, it seems to have no foundation in 

 theory except as the result of metCDrological causes, in which 

 case we can hardly hope for more success in dealing with it 

 than in predicting the weather on which it depends. For 

 further improvement in our knowledge of this subject we must 

 look to continued observation at a number of points carried on 

 for this express purpose, .and so conducted as to eliminate, if 

 possible, all systematic errors. If, as seems probable, the co- 

 efficients—at least that of the annua! term — partake of the erratic 

 nature of meteorological phenomena, it will be necessary to keep 

 this work up perpetually. 



A plan is under discussion for establishing four permanent 

 latitude stations on the same parallel of latitude, at intervals of 

 90° in longitude as nearly as may b:. These will presumably be 

 equipped with identical instruments of the most approved form, 

 and the same stars employed at all of them. Until this plan, or 

 some modification of it, is in working order — and probably for 

 some time after — careful determinations at other points will con- 

 tinue to furnish valuable data, especially in settling the question 

 of progressive changes, local or otherwise. 



The instrument hitherto employed in special observations for 

 this purpose is the zenith telescope. The possibility of deter- 

 mining latitude by measurement of the small difference of zenith 

 distance of two stars properly situated — one culminating north, 

 the other south of the zenith — was pointed out by Ilorrebow in 

 his Alrium Astronomintt in 1732. (Wolf, " Geschichte der As- 

 tronomical," p. 608.) Possibly he may have made a practical 

 application of ihe principle ; if so, any account of it has escaped 

 my notice. The method, however, was employed by Father 

 Hell— otherwise not unknown to fame — in determining the lati- 

 tude of his transit of Venus station at Wardoehume in 1 769. He 

 appears to have been unacquainted with Horrebow's previous 



NO. 1245, VOL. 48] 



suggestion, and determined his latitude in this way, as he says, 

 from necessity. 



The idea seems to have lain dormant until about 1834, when 

 it was hit upon independently by Talcott in Americ, and 

 Pond in England. The latter, in employing the zeniih tele- 

 scope — which had then been recently mounted at the Royal 

 Observatory for the special purpose of observing 7 Draconis — ■ 

 found that a fifth magnitude star passed the meridian thirty 

 minutes later at nearly the same distance on the opposite side 

 of the zenith. 



By observing these two stars, reversing the instrument 

 between them, he found certain advantages now well known to 

 be inherent in the method. [Phil. Trans., vol. cxxiv. p. 209 ) 

 Pond states that the same method may be employed with 

 Altazimuths, and other portable instruments, but the communi- 

 cation appears to have attracted no attention, and apparently he 

 made no attempt to develop it farther. 



In striking contrast is the immediate success which attended 

 the employment by T.ilcott of an instrument constructed to carry 

 out this principle. The first practical application of it was in 

 18 34, in the survey of the northern boundary of Ohio, {jfounial 

 Franklin Institule, October, 1838.) Its merits were very 

 promptly recognised by the officers of the U.S. Coast Survey, 

 where it received a number of modifications and improvements 

 suggested by experience, making it practically the instrument 

 which we have to-day. It was many years, however, before it 

 came into use to any considerable exLent on the eastern side of 

 the Atlantic. 



To America undoubtedly belongs the honour of practically 

 introducing this important improvement in latitude determina- 

 tion. 



But although Americans practically introduced the instrument 

 to the world, it was reserved to the Germans to teach us how 

 to use it. It is due in great measure to refinements and im- 

 provements devised by German observers and instrument 

 makers that the probable error of a single determination is 

 now •12" or Ts", instead of three times these amounts, with 

 which we were formerly satisfied. The essential features of 

 this instrument are the micrometer and the level. Unless these 

 are of a high degree of excellence first class re>ults cannot be 

 obt.ained ; especially is this true of the level, of which two are 

 commonly employed with the best class of instruments. Only 

 those who have experienced it are aware how difficult it is to 

 procure levels of the necessary quality. Moreover, changes of 

 form are liable to occur, rendering what was a good level 

 worthless. The method so frequently employed by determining 

 the value once for all, and continuing to use it for years without 

 farther examination will not answer here. 



This uncertainly of the level has led to devices for dispensing 

 with it. One of these, which seems promising, is the floating 

 Zenith telescope, invented by Fathers Hagan and Fargie. In this 

 instrument the telescope,with its accessories, floats on the surface 

 of a trough of mercury, the trail of the star as it crosses the field 

 being recorded on a photographic plate, which may be measured 

 at leisure. Possibly a way may be formed for making these 

 exposures automatically, thus furnishing means for keeping a 

 record continuous in so far as absence of daylight and of clouds 

 will permit. With four stations established as described above, 

 equipped with automatic instruments, data will be rapidly accu- 

 mulated for settling the questions still remaining doubtful. It 

 will not, however, be a work of merely one, two, or three, but 

 of many years. 



Is it too much to hope that within five or ten years we may 

 see some such system as this in full and successful operation ? 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 

 A Parliamentary paper hasjust been issued in which is given 

 an abstract of returns furnished to the Departraen' of Science 

 and Art, showing the manner in which, and the extent t > which 

 the councils of counties and county boroughs in England and 

 Wales, and the county councils, town councils, and police com- 

 missioners of police burghs are devoting funds to the purposes 

 of science, art, and technical and manual instruction. The 

 returns were made by these bodies in response to a letter sent 

 to them in December, 1892, by the Education Department. 

 Much of the information contained in them was noteJ in these 

 columns on August 2S (p. 404). It is remarked in the present 

 returns : "A noticeable feature with regard to the work of the 



