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HISTORY OF PHYSICAL ASTRONOMY.

positions of the planet as determined hy these observations shewed thatthe effect of parallax was insensible; whence it was inferred that at theutmost it did not exceed 25". This gave 10" as the greatest valuewhich could he assigned to the solar parallax. Cassini fixed it at 9".5,a result which formed a very important approximation to the true value.When Lacaille was at the Cape of Good Hope , he made observations ofMars for a similar purpose. A comparison of his results with thoseassigned by similar observations in Europe gave 10". for the amountof the sun’s parallax.

The transits of Venus across the sun’s disk in the years 1761 and 1769supplied astronomers with a more accurate method of determining thesolar parallax than that to which we have been just alluding. Thismethod was first pointed out hy Halley, who, in 1716, suggested toastronomers to carry it into effect on the occasion of the approachingtransit of the planet, earnestly imploring them not to neglect so valuablean opportunity of determining so important an element. Astronomersdid not fail to appreciate the value of Halley’s proposal; and on theirrecommendation the principal Governments of Europe fitted out expe-ditions to various parts of the world, both in 1761 and 1769, for theexpress purpose of observing the transit of the planet. The transit of1761 w r as imperfectly observed, and the result did not prove satisfactory.The transit of 1769, however, was observed with complete success. Acomparison of the results assigned by the various observations gave 8".7and 8".5 as the most probable limits within which the parallax is confined.Delambre assigns 8".6 as the most accurate value, a result which placesthe sun at the distance of 95 millions of miles from the earth. When weconsider the ingenuity of the method employed in arriving at this deter-mination, and the refined nature of the process by which it is carried intoeffect, we cannot refrain from acknowledging it to be one of the noblesttriumphs which the human mind has ever achieved in the study of physicalscience.

A comparison of the value of the solar parallax as assigned by observa-tions of the planet Mars , with that obtained by the more accurate methodof the transit of Venus, did not lead to so close an agreement as mighthave been expected from the advanced state of astronomy. With theview of arriving at a more satisfactory result by the former of thesemethods, the late Mr. Henderson made observations of Mars at the Capeof Good Hope, on the occasion of the opposition of the planet in 1832.Simultaneous observations of the planet were made at Greenwich, Cam-bridge, and Altona. A discussion of all the observations gave for thesolar parallax a mean value equal to 9". 125. This result forms a nearerapproximation to the generally acknowledged value, than either of thoseassigned by Cassini or Lacaille, but still it has not altogether satisfied thedesire of further research on the subject. A method for determiningthe solar parallax by means of observations of Venus and Mars hasrecently been proposed by Dr. Gerling of Marburg. A suggestion whichwas made to the Government of the United States to lend its co-operationin carrying this method into effect, has been favourably received; and, in pur-suance of this object, an expedition has been despatched to the State of Chili,in South America , in the month of July of the present year*. It is con-templated to observe Mars at the oppositions of 1849-52; and Venus at

* 1849 .