23 



The Sun's mean horizontal equatorial parallax has since been computed, by 

 E. J. Stone, Esq., from observations made at the Roj^al Observatory, Green- 

 wich, and the Government Observatory, Williamstown, Victoria, during the 

 last opposition of Mars, in 1862. The mean result is 8" '932, with a probable 

 error of 0-032. 



It is clear from these resultants that the three astronomical methods, even 

 if we select the most trustworthy, differ by three or four millions of miles — 

 three or four per cent, of the whole quantity. The commonly received distance 

 of the Sun is based upon Encke's profound discussion on the observations 

 made at the last two transits of Venus. Encke decided from the weights of 

 che observations, discussed on the mathematical principle of least squares, that 

 the probable error in the Sun's distance, as given by the transits, does not 

 exceed l-230th of the whole quantity. Astronomers have also reason to believe 

 that the adopted value of observation is correct within 1- 1800th of the whole 

 quantity. Foucault's is confident that with his improved apparatus he can 

 banish all errors greater than l-6000th of the whole quantity. It follows 

 then, that one of these three elements, either the velocity of light, aberration, 

 or the Sun's distance must be in error to the extent of three or four per cent. 

 Which of the three must be changed ? 



This question remains to be answered by astronomers at the nest two transits 

 of Venus — December the 8th, 1874, and December the 6th, 1882* —for which 

 purpose England, France, Kussia, and America have already taken some pre- 

 liminary steps for sending out scientific expeditions. In a paper read April 

 8th, 1857, before the E,oyal Astronomical Society, by the Astronomer Koyal, 

 rules are laid down, instruments described, and localities fixed upon — amongst 

 the latter Van Diemen's Land is very favorably mentioned, but Professor Airy 

 expresses a doubt whether the longitudes of any of the stations named, except- 

 ing those in Europe, are yet known with sufficient accuracy. Sir Henry 

 Young (our late Governor) on being made acquainted with the opinion of the 

 Astronomer Royal, at one of our Monthly Meetings, suggested that I should 

 confer with Lieut. Brooker, and try to remove the doubt from the mind of 

 Professor Airy. The result of this interview with Mr. Brooker was the de- 

 ductions of the following resultant from Captain Kay's observations, made 

 for the Magnetic Observatory, one copy of which was forwarded by agreement 

 to the Royal Astronomical Society, and another copy to the Admiralty, 17th 

 December, 1861 : — 



Captain Kay's communication to the Royal Society of Van Diemen's Land, 

 in 1852, of the geographical position of the magnetic observatory, Hobart 

 Town, gives such an elaborate detail of the means adojited for ascertaining its 

 latitude and longtitude, that with verification by chronometric measurements, 

 and — what may be possibly thought — accidental accordance from the measure- 

 ments obtained between the Cape of Good Hope and Hobart Town, it may be 

 fairly and reasonably assumed that the longitude of the observatory is (if not 

 correctly known) but very little in error — less than half a mile ; for in his 

 tabulated record of the niimerous observations by eclipses of the sun, by 

 Jupiter's satellites, by moon-culminating stars, and by chronometric measure- 

 ments, extending over a period of several years, by several observers, the 

 extreme range of difference amounting to only 3| miles. However, by the 

 means of eight general results, it is reduced to less than half a mile of the 

 probable truth. To attain a greater certainty would necessitate the establish- 

 ment of a fixed observatoiy,with the best instruments,t and careful observations 

 by experienced scientific observers." 



Her Majesty's ship "Herald," Captain Denham, was in these waters on a 

 scientific cruise in the year 1859, and in December of that year was stationed 

 at Garden Island, Sydney. The result of the observations taken at that time 

 by the officers of the "Herald" gave the longitude for Garden Island 

 lOh, 5m. l-9s. This would make the longitude for Hobart Town 9h. 49m. 28 -83., 

 whereas Captain Kay's observations give it 9h. 49m. 29'6, making a difference 

 of 8-10 of a second, which may be accounted for by the observations of the 

 "Herald" being referred to the harbor, and Captain Kay's to the observatory. 

 Trifling as 8-10 of a second may appear, unless accounted for, it would entail a 

 considerable error in the sun's distance. The solar pai-aUax is only about eight 

 seconds and a half, and an error of 1-10 of a second includes an error of more 

 than a million of miles in the sun's distance, in which a con-ection of three per 



* A reconnaissance of Wilk's Land is also required to be effected, included between 

 Sabrina Land and Repulse Bay, occupying an extent of about 400 miles. To secure 

 observations both of ingress and egress in this track is considered indispensable. 



+ The telescope used at the magnetic observatory was a portable achromatic by 

 Dolland, 42 inch focal length, and 2^ inch apperture, on a pillar and claw stand. 



