9 



ON THE TEANSIT OF MERCUEY OVER THE SUN'S 

 DISC, NOV. 5th, 1868. 



By Francis Abbott, F.R.A.S., &c. 



I lay on tlie table a diagram in whicTi is embodied the ele- 

 ments, and a full explanation, of tbe transit of Mercury, with 

 the true path of the planet over the sun's disc on the 5th of 

 November, 1868. The observations were male with a five 

 foot equatorially mounted achromatic telescope. Other obser- 

 vations were also made at the time, and near the same place, 

 with a small Oregorian reflector. The day was fine, and the 

 passage of the planet over the sun's disc was observed as a 

 well-defined round black spot. When the true limb of the 

 planet reached the true limb of the sun, the light being 

 cut off, a fine dark ligament appeared to connect the limb of 

 the planet and the sun, showing the true time at first contact. 



The Hobart Town time of transit was computed from the 

 Nautical Almanac ; the observations made with a half second 

 chronometer, and when compared with the normal clock it was 

 found to be remarkably close, I scarcely think one second in 

 error. 



The sun's limb was well defined, and the tremor by no means 

 considerable. Some measures were made of the difference of 

 right ascension of the Sun and Planet, they have been corrected 

 for refraction, and for the motion of the sun, and are compared 

 with the elements given in the nautical almanac, as shown in 

 the diagram. In the observation I saw neither central 

 bright spot, halo, nor satellite. My attention was directed 

 to time determinations for ascertaining longitude, rather 

 than to any peculiarities that might be presented during the 

 transit. 



The Royal Astronomical Society are in full possession of all 

 the means employed by Commander Kay for determining the 

 geographical position of the Magnetic Observatory here, the 

 whole series of observations consisted of moon culminating 

 stars, of which 41 sets from both limbs of the moon were 

 observed on the meridian, with an excellent transit instrument 

 by Troughton and Simms. The solar eclipses, and the eclipses 

 of Jupiter's satellites were observed with a telescope made for 

 the purpose by Jones. When the distance of any two given 

 places is not very great, the longitude is determined with 

 accuracy by transmission of chronometers, for which purpose 

 Captain Kay availed himself of the well known positions of 

 the observatory at Parramatta and Fort Macquarie, Sydney 

 Harbour ; by this method no less than 76 chronometers were 



