1857

“In the Report to the Visitors, when on the subject of the Altazimuth, the following paragraph occurs:  ’I alluded in a preceding section to the cutting away of a very small portion of one of the rays of the three-armed pier which carries the Altazimuth.  The quality of the brickwork is the best that I have ever seen, and not a single brick was disturbed beyond those actually removed.  Yet the effect was to give the Altazimuth an inclination of about 23”.  This inclination evidently depends on the elasticity of the brickwork.’—­With reference to the new S.E.  Equatoreal the Report states that ’The support of the north or upper end of the polar axis has been received, and is planted within the walls of the building in a position convenient for raising it to its ultimate destination.  It is one piece of cast-iron, and weighs nearly 5 tons.’—­Small changes as previously mentioned had been noticed with regard to the Zero of Azimuth of the Transit Circle, and the Report states that ’In regard to the Azimuth of the Transit Circle, and the Azimuth of its Collimator, Mr Main has brought together the results of several years, and the following law appears to hold.  There is a well-marked annual periodical change in the position of the Transit Circle, the southerly movement of the eastern pivot having its minimum value in September, and its maximum in March, the extreme range being about 14 seconds; and there is a similar change, but of smaller amount, in the position of the Collimator.  I cannot conjecture any cause for these changes, except in the motion of the ground.  There is also a well-marked connection between the state of level of the axis and the temperature.  The eastern pivot always rises when the temperature rises, the extreme range being about 6 seconds.  I cannot offer any explanation of this.’—­Under the head of Extraneous Works the Report states that ’The British Government had for some years past contributed by pecuniary grants to the preparation of Prof.  Hansen’s Lunar Tables.  In the last winter they undertook the entire expense of printing a large impression of the Tables.  The reading of the proof-sheets (a very considerable labour) has been effected entirely at the Observatory.  I may take this opportunity of stating that the use of these Tables has enabled me, as I think, incontestably to fix the capture of Larissa to the date B.C. 557, May 19.  This identification promises to prove valuable, not merely for its chronological utility, but also for its accurate determination of an astronomical epoch, the point eclipsed being exactly known, and the shadow having been very small.’—­In April I gave a lecture to the Royal Astronomical Society on the methods available through the next 25 years for the determination of the Sun’s parallax.—­Dr Livingstone’s observations for African longitudes were computed at the Observatory.—­The Admiralty enquire of me about the feasibility of adopting Piazzi Smyth’s construction for steadying telescopes