1877

“In April of this year I was much engaged on the subject of Mr Gill’s expedition to Ascension to observe for the determination of the parallax of Mars at the approaching opposition of that planet.—­A large Direct-vision Spectroscope has been quite recently made by Mr Hilger under Mr Christie’s direction on a new plan, in which either great dispersion or great purity of spectrum is obtained by the use of ‘Half-prisms,’ according as the incident pencil falls first on the perpendicular or on the oblique face.  In this Spectroscope either one or two half prisms can be used at pleasure, according to the dispersion required, and there is facility for increasing the train to three or four half-prisms, though the dispersion with two only is nearly double of that given by the large ten-prism Spectroscope.  The definition in this form of Spectroscope appears to be very fine.—­At the end of May 1876, spectroscopic determinations of the Sun’s rotation were made by observations of the relative displacement of the Fraunhofer lines at the east and west limbs respectively.  The results are in close agreement with the value of the rotation found from observations of Sun-spots.  A similar determination has also been made in the case of Jupiter, with equally satisfactory results.—­An Electrometer on Sir William Thomson’s plan, for continuous photographic registration of atmospheric electricity has been received from Mr White of Glasgow.  It was mounted in December.—­The computation of the photographic records of the barometer from 1854 to 1873 has so far advanced that we can assert positively that there is no trace of lunar tide in the atmosphere; but that there is a strongly marked semi-diurnal solar tide, accompanied with a smaller diurnal tide.  We are at present engaged in comparing the barometric measures with the directions of the wind.—­Regarding the distribution of the printed observations:  There is no extensive wish for separate magnetic observations, but general magnetic results are in great demand, especially for mining operations, and to meet this a map of magnetic declination is furnished in the newspaper called the ’Colliery Guardian.’—­As regards the operations for the Transit of Venus:  The computing staff has by degrees been reduced to two junior computers within the Observatory; and one or two computers external to the Observatory, who are employed on large groups of systematic calculations.  The principal part of the calculations remaining at the date of the last Report was that applying to the determination of the geographical longitudes of fundamental stations.  At the moment of my writing, the last of these (the longitude of Observatory Bay, Kerguelen) is not absolutely finished:...  The method of determining the geographical longitude of the principal station in each group by vertical transits of the Moon has been found very successful at Honolulu and Rodriguez.  For stations in high south latitude, horizontal transits are preferable.—­As regards the Numerical