CAPE OF GOOD HOPE OBSERVATIONS. 



EDITOKIAL NOTICE. 



OFFICE U. S. NAVAL ASTRONOMICAL EXPEDITION. 



The following note appears on page 106, vol. XX, Memoirs of the Koyal Astronomical 

 Society: London, 1851. 



"Mr. MACLEAR forwarded to England two copies of the observations of Mars made with the 

 8^-feet Equatorial at the Cape of Good Hope. In one all the phenomena are noted, but unreduced; 

 in the other, the observations are fully reduced, except that the correction for parallax has not 

 been applied to the planet. The specific object of these observations was to supply materials 

 for determining the parallax of Mars in conjunction with Lieutenant GILLISS'S expedition to 

 Chile, and a copy will be forwarded to America for that purpose. It has therefore been thought 

 unnecessary to print the details here, while the materials for deducing a place of Mars were 

 considered to be too valuable to be omitted or postponed." 



On application to the Secretary of the society, the observations embraced in the following 

 pages, together with the description by Mr. Maclear of the micrometer, the equatorial adjust- 

 ments, the particulars relating to the observations, the rate and error of Barraud's sidereal 

 clock No. 1190, and the barometers, were immediately forwarded to me for publication. As 

 many who will receive this volume may not have access to the publication above referred to, in 

 order that the contribution of Mr. Maclear may be as complete as possible, it is deemed proper 

 to reprint here his account of the instrument. 



MR. MACLEAR'S DESCRIPTION OF THE EQUATORIAL INSTRUMENT. 

 [From vol. XX, Memoirs of the Royal Astronomical Society: London, 1851.] 



" The instrument is the work of MERZ & SON, of Munich. It diifers little, except in dimen- 

 sions, from those made by the same artists for the observatories of Pulkova and Washington, 

 and is nearly identical with that which belongs to the Rev. W. E. DAWES. The telescope tube 

 is of wood, veneered with mahogany. It is very firm, as will be shown presently. The object- 

 glass is nearly seven inches in diameter, and about 8 feet focal length. It is provided with 

 two micrometers and eleven eye-pieces. The powers of the eye-pieces belonging to the position 

 micrometer are 123, 161, 273, 347, and 464, calculating them from the diameters of the images; 

 on the supposition that the clear aperture of the object-glass is 6.9 inches. The power of the 

 double annular micrometer is 64, and of the five Huyghenian eye-pieces, 86, 128, 200, 302, and 

 458. There is a prismatic piece for viewing objects near the zenith, on which the eye-pieces fit. 



" The divided circle of the position micrometer is four inches in diameter. Each degree is 

 divided into four parts, and, by means of two opposite verniers, to single minutes. The 

 divisions are on the plane of the circle. The counting scale of the micrometer screw is outside, 

 an essential arrangement for 'dark field' measurements. One revolution of the micrometer 

 screw carries the wire over 26". 0697, and its total range is 60 revolutions. There is a reflector 

 in the tube for the ordinary method of 'down-tube' illumination, and a double lamp appar- 

 atus near the eye-piece for 'dark field' illumination. The latter is almost exclusively 

 employed. 



