204 



NATURE 



{yaii. 2, 1879 



currents would certainly not have as yet disappeared, but 

 would still be the chief agents in determining the move- 

 ment of the ocean at great depths, supposing that the 

 earth were completely covered by an ocean of the uniform 

 depth of 4,000 metres. 



The interruption by continents and islands of irregular 

 form will contribute to weaken the effect of these former 

 states of motion, not so much through the increased 

 friction on the ocean-bed as through the reflex currents, 

 which arise everywhere, crossing and impeding one 

 another. But it must be observed after the above nume- 

 rical proof of the extremely slow spread of local altera- 

 tions of motion over the interior mass, that the difficulties 

 of an exact computation must not be shirked, on account 

 of the traditional expression: "Friction quickly uses up 

 all these velocities." 



It would be possible to determine by observations 

 whether effects of former movements are still present in 

 the ocean. There would be required for this purpose 

 only comparative current-observations at the most 

 varied depths, to be applied in the central parts of the 

 great equatorial currents and of the region of calms. 

 Yet, however, we dare not hope to be able to detect 

 small remnants of interior motion with the same certainty 

 with which the effect of the former high temperature of 

 the earth, which disappears according to the same law, 

 could be detected by subterranean observation of tem- 

 perature, were one able to penetrate deep enough with 

 the thermometer into the earth's crust. 



The above computations give us also an idea how dis- 

 tant must be the time of the initial state. What a long 

 time, for example, must we imagine the trade winds to 

 have been blowing with their present extent and strength 

 in order to be justified in assuming that the present state 

 of motion of the equatorial currents is steady. For that 

 about 100,000 years are needed, supposing we postulate a 

 mean depth of 4,000 metres and do not take into account 

 the deadening influence of continents and islands which 

 must somewhat diminish that number. Every initial 

 state, whatever it may have been, vanishes finally, and 

 gives way to a steady state, only the time varies which is 

 required to diminish the originally arising velocity to any 

 required degree of smallness. 



OUR ASTRONOMICAL COLUMN 



The Melbourne Observatory. — The thirteenth 

 official Report of the Board of Visitors of the Melbourne 

 Observatory, with the annual statement of the Govern- 

 ment Astronomer, is before us. Mr, Ellery reports that 

 the new building to contain the magnetical and meteoro- 

 logical instruments registering continuously by photo- 

 graphy is completed. The staff of the Observatory now 

 consists of the director, with a chief assistant (Mr. White) 

 and three junior assistants. The transit-circle is found 

 to be inadequate for modern requirements, and the Boai-d 

 of Visitors lay stress upon the necessity of providing an 

 instrument of greater pretensions, to enable Melbourne 

 to co-operate effectively with European and American 

 observers ; the Sydney Observatory being already in pos- 

 session of a very superior meridian-instrument, and one 

 having been ordered, it is understood, for the observa- 

 tory under the direction of Mr. Todd at Adelaide, it is 

 hoped that a new transit-circle may soon be provided for 

 Melbourne, and it is suggested that the necessary appro- 

 priation, about 1,200/., might be made in two annual votes, 

 as two years will be required for the completion of the 

 instrument. 



The great reflector, though reported to be working 

 satisfactorily, the mirrors retaining an excellent polish, 

 and no marked signs of deterioration being visible, is 

 occasionally subject to trifling derangements of its 

 mechanism. Unfortunately the publication of the work 

 with this instrument, the drawings of nebulas, has been 



delayed by the loss of the gentleman who copied the 

 drawings on stone. The drawings, however, now only 

 require printing, and their publication is not likely to be 

 long retarded- Mr. Ellery refers to the miscellaneous 

 observations made during the year to which his report 

 relates (to June 30, 1877), including observations of 

 D' Arrest's comet of short period, determination of 

 positions of stars used by Mr. Gill during his expedition 

 to Ascension, measures of southern double stars and of 

 the polar and equatorial diameters of Mars, and of 

 Saturn's ring. With regard to the use of the great 

 reflector it is mentioned, "Out of 326 available nights 

 150 were unfitted for observation from unfavourable 

 weather, bright moonlight interfered on 32, while 49 

 were occupied with visitors, which, together with about 

 20 nights during which the telescope was under repair, 

 or which were unavailable from other causes, left only 

 75 nights upon which observations could be made." 

 From the observations made during the year upon ']'] 

 of the smaller nebulae in Sir John Herschel's "General 

 Catalogue," it is gathered that while the actual aspect of 

 many conforms precisely with Herschel's description, 

 others are so considerably changed as to be only recog- 

 nisable by their position. The only change detected in 

 the great nebula about r) Argus, since the drawing in 

 March, 1875, has been " a break or separationjn one of 

 the branches on the preceding side." 



Observations of the satellites of Uranus were made on 

 sixteen nights, and on the same number of nights the 

 satellites of Mars (the announcement of the discovery of 

 which had been telegraphed to Mr. Ellery by Sir George 

 Airy) were unsuccessfully sought for ; the failure to find 

 these objects with certainty and ease Mr. Ellery considers 

 "somewhat unaccountable," but the reader will hardly 

 need to be reminded that there are other cases where 

 the large reflectors have not proved so adequate for work 

 as the large refractors : sooner or later, at Melbourne or 

 elsewhere, we hope to see a large instrument of the latter 

 class applied to the survey of the southern heavens : the 

 real astronomical work in the northern hemisphere, the 

 more precise micrometrical measures and more delicate 

 observations falling to the task of the practical astro- 

 nomer, have been, as yet, pre-eminently due to the use 

 of the refractor. 



BiELA's AND Halley's Comets.— There are near 

 approximations between the orbits of these bodies not 

 far from points which were first roughly indicated by 

 Littrdw, in a communication to the Vienna Academy in 

 1854, entitled " Bahnnahen zwischen den periodischen 

 Gestirnen des Sonnensystemes." In heliocentric longi- 

 tude 39° 25' (equinox of 1836) the distance between the 

 two orbits is 0*032 (the earth's mean distance from the 

 sun = i), and in 200^ 51', the distance is as small as 

 o'oii. At the former point the true anomaly of Halley's 

 comet is - 94=^ 9', with the elements of 1 836, and that of 

 Biela's - 71° 17' ; at 200" 51' the true anomaly of Halley's 

 is + 104" 59', and of Biela's -|- 90° 2' ; we see then that 

 on the last return of Halley's comet to these parts of 

 space, though its orbit approached so near to that of 

 Biela's, there was no near approximation of the two 

 bodies. It will be remembered that Biela's comet also 

 passes very near to the orbit of Tempel's comet 1866 I., 

 and consequently to the track of the November meteor- 

 stream. 



GEOGRAPHICAL NOTES 



Among the geographical notes in the January number 

 of the new periodical issued by the Royal Geographical 

 Society we find some interesting information regarding 

 the work to be done by Mr. Keith Johnston's East African 

 Expedition. He is instructed to gather data for con- 

 structing as complete a map as possible of the route, and 

 to make all practicable observations in meteorology, 



