124 LECTURES 



being within two or three degrees of the sun, this distance will differ 

 but little from E V. The horizontal parallax of Venus will then be 

 readily found. - 



But the parallax of Venus and the sun will be inversely as their 

 distances. By Kepler's Third Law we should have — 



In this proportion the terms are all known except the last, and 

 hence we have the sun's parallax. 



The letter of Dr. Gerling to Lieutenant Gilliss was in anticipation 

 of the publication of his views in Germany. Lieutenant Gilliss 

 promptly brought the subject to the notice of our ablest astronomers, 

 and, with their hearty concurrence, enlisted Congress, the government 

 and the Smithsonian Institution in the enterprise. 



The manner in which the expedition was undertaken and carried 

 out reflects the highest credit upon the ability and untiring energy 

 of Lieutenant Gilliss. The very moderate appropriation of $5,000 

 was authorized by Congress to enable the Secretary of the Navy to 

 defray the expenses of the expedition. Furnished with instruments 

 fitted expressly for the expedition, viz : two telescopes, a larger and 

 smaller, equatorially mounted ; a meridian circle, of large size and 

 exquisite finish; clock and chronometers; and two assistants, Messrs. 

 MacRae and Hunter, Lieutenant Gilliss, by order of the Secretary of 

 the Navy, repaired to Santiago de Chile, and erected his observatory 

 so as to commence observations in December of 1849, which were con- 

 tinued till September of 1852. By previous arrangement, simultaneous 

 observations on both Venus and Mars were to be made at the Wash- 

 ington observatory, under the direction of Lieutenant Maury. It was 

 expected also that other observers, both in this country and Europe, 

 would, as far as possible, co-operate in a work in which all astrono- 

 mers were alike interested. 



On the return of the expedition the vast mass of observations were 

 placed in the hands of Dr. B. A. Gould, jr.^ for reduction and discus- 

 sion. The well known ability of Dr. Gould gave assurance that 

 nothing would be wanting to insure the most accurate results. The 

 laborious undertaking has been accomplislied, and the result published 

 in the third volume of the Naval Astronomical Expedition. The 

 parallax thus obtained from purely American observations, and by a 

 method entirely new, at least so far as the observations on Venus are 

 concerned, agrees very nearly with those obtained by the transit of 

 Venus, viz: 8". 5. If we cannot say that our own astronomers have 

 attained a closer approximation to the truth than had before been 

 reached, which could hardly be expected, we can say that by a new 

 and independent method they have added a most valuable confirma- 

 tion to results before obtained. It is not too much to say that this 

 enterprise, in its conception and consummation, reflects honor upon 

 "the scientific chaiacter of our country, and will be highly appreciated 

 wherever astronomical science is cultivated. 



The next two transits of Venus will be on December 8, 1874, and 



