OF THE EXPEDITION. Xvil 



of condition derived from both transits.' But, compared with the transit of 1769, he speaks of 

 the next two transits those of 1874 and 1882 as unfavorable for this purpose, presenting 

 'disadvantages which even the perfected state of instruments will not counterbalance;' and in 

 view of the extremely rare occurrence of these phenomena, and of the unfavorable character of 

 several succeeding ones, he intimates that, for three thousand years the transit of Venus of 

 1769 may remain as the chief means from which we are to derive our knowledge of the actual 

 dimensions of the solar system. At any rate, it will not be before 1874 that astronomers can 

 avail themselves of a transit of Venus for even an imperfect verification of Encke's parallax; 

 and centuries will elapse before the recurrence of one as favorable for the purpose as that of 

 1769. But even for the expiration of these twenty-six years the rapid advance of science will 

 not permit them patiently to wait. The progress of astronomy, and the interests of navigation, 

 require an earlier confirmation or re-determination of a quantity so important ; and astronomers 

 will watch with deep interest the accumulation of observations which will bear upon this point, 

 and which, if even less perfect in themselves separately, will yet finally, by their numbers, and 

 the variety of circumstances under which they shall be made, be entitled to weighty considera- 

 tion. 



"Two methods have been suggested one by observations on Mars, which has been only par- 

 tially tried, and with but partial success ; the other, the one which it i^ now proposed to attempt, 

 and which offers a decided advantage in the greater proximity of Venus to the earth. It has 

 been proposed to astronomers from a respected and distinguished source. It comes to us re- 

 commended by other names of high authority, and by two learned bodies of our own land 

 the American Academy of Sciences, and the American Philosophical Society. 



"But, although the proposed expedition of Lieut. Gilliss is for carrying out chiefly the latter 

 method, it need not, and will not, preclude the other. And the various other observations he 

 will be able to make during the two years he proposes to devote to this work, will furnish a rich 

 and valuable series; for which alone, entirely apart from the consideration of the main design, 

 astronomers and scientific men in other spheres, as appears from the accompanying correspond- 

 ence, have applauded and warmly commended his purpose. 



" But the main design itself is not without the best prospect of success. The committee are 

 informed by practical astronomers, that, taking for example the proposed meridian observa- 

 tions, with the nicely constructed instruments of the present day, and good observers, 0".7 would 

 be a large estimate of the probable error of each observation. The probable error, then, of the 

 solar parallax, deduced from two such observations at stations as distant as Washington and 

 the proposed position in Chile, will be 0".3 to 0".7, according to the distance at the time of Venus 

 from the earth. And the error of the result, from an accumulated number of such observations, 

 may be rendered very small, provided no constant uneliminated error remains, which will be 

 common to them all. But the liability to such errors can be greatly diminished by the skill of 

 the observers, and their careful attention to all the circumstances which may affect their pro- 

 blem. And this liability to error will be still further diminished by combining the results of 

 the meridian series with those from the extra-meridional observations contemplated, and which 

 will have been made Bunder far different and more variant circumstances. These common 

 errors, the computation of probable error does not include ; and it may be doubted whether the 

 resulting parallax of 1769 is wholly free from the effects of such. Astronomers aim, ever, that 

 their results shall be deduced from observations as extended, made under circumstances as vary- 

 ing, and with means as different as possible. And, in this respect, the determination of the 

 solar parallax, as yet, stands almost alone as the result almost of a single phenomenon. 



" But the committee do not deem it necessary to enter further into a subject which is so fully 

 discussed in the interesting correspondence attached. They are fully impressed with the im- 

 portance of the object in view ; and as the sum required will not exceed $5,000, they report an 

 amendment to the naval appropriation bill to enable the Secretary to accomplish it. All of 

 which is respectfully submitted." 

 D 



