x ORIGIN AND OPKRATIONS 



be preferable to Chiloe, if it be probable that five observations of the planet may be obtained at 

 the first as frequently as four can be made at the second station. You will, of course, under- 

 stand me as meaning Valparaiso only as an example for your letter, and that the same principle 

 of reasoning must i,e applied in the selection of any neighboring locality. Yet I repeat, a 

 decision of this (i uestion must rest wholly with you in America, where you, undoubtedly, have 

 at command nearer and more copious sources of information respecting the climate than we, and 

 more particularly than myself, who have never given thorough attention to subjects of this 

 nature But, whatever may be your decision, I can but congratulate myself that by the com- 

 munication of my views relating to observations on Venus, I may prove the remote cause of so 

 great and important an enterprise. 



" I greatly rejoice that you have reasonable expectation to obtain a good circle for the Expe- 

 dition, to serve you in the declination observations ; for I am confident such an instrument will 

 give glorious results. And that a valuable clock is also probably at command, is a subject of 

 much congratulation. Should you find the opinions of others useful in enabling you to pro- 

 cure these instruments, that of Gauss, which was copied into my last letter for you, may prove 

 of considerable weight. 



" I fully concur with you in the belief that the value of the micrometrical observations will 

 be much increased if they are made at the two stations on the same stars at periods as nearly as 

 possible simultaneous ; and to this end I shall forward to you, at the earliest possible day, a 

 list of stars selected from the Berlin charts. But I do not immediately perceive any great 

 benefit likely to accrue from a comparison of the right ascension observations made on the meri- 

 dians of the respective parallels, unless, as intimated, we should succeed in obtaining extra- 

 meridional observations of Venus whilst the sun is above the horizon, and simultaneous measures 

 of the eastern and western hemispheres may be combined. Irrespective of the last contingency, 

 I cannot but regard your views directly to the point, and am satisfied we should endeavor to 

 make a multitude of observations in the northern and southern hemispheres, if possible, abso- 

 lutely simultaneous. By so doing we shall especially obviate a difficulty of an important char- 

 acter, but one which did not occur to me until after I had despatched my last letter, viz : 

 uncertainty in the time of the determination ; for, as the parallax of Venus when approaching 

 the horizon is expressed by the greatest numbers, so also at that time its variation is the most 

 rapid, and the necessity for accurate knowledge of the instants of observation proportionately 

 greater. The simultaneous observations you propose will certainly remove this disadvantage." 



Meanwhile, the early letters of Dr. Gerling had, with my own, been submitted to the Ameri- 

 can Philosophical Society and the Academy of Arts and Sciences, to Professors A. D. Bache, 

 Benjamin Peirce, S. C. Walker, Elias Loomis, and others eminent for mathematical and astro- 

 nomical attainments, from each of which there was solicited impartial investigation of the 

 method and proposed mode of carrying it into effect, and a recommendation of the Expedition to 

 the favorable consideration of the government, should they believe it likely to obtain useful 

 results. Prof. Bache replied : 



" As was no doubt expected, the searching examination to which your proposal has given 

 rise has nearly exhausted the various bearings of the problem. The importance of the inquiry 

 is very generally admitted. In discussing the question of the probability of obtaining new data 

 for the solar parallax which shall diminish the probable error of the value obtained from the 

 transit of Venus, there are different shades of opinion expressed. I do not see, however, that 

 the two reasons which strongly favor Dr. Gerling' s method are met by any opposing argu- 

 ments. The large number of observations upon which results may be founded, and the inde- 

 pendence of the new method with that formerly used, are, indeed, striking features in this 

 method. Independent methods give the best confirmation of results, or show errors beyond the 

 accidental error deduced from calculation of observations which are all made by the same 

 method. 



'There can be no doubt that the instruments to be employed should be of the class used in 



