X ORIGIN AND OPERATIONS 



preferable to ChihV. it" it bo probable that five observations of the planet may be obtained at 

 iw frequently as four can IT made at tin- second station. You will, of course, under- 

 stand nie as moaning Valparaiso only as an example tor your letter, and that the same principle 

 of reasoning must lie a|)]lie<l iti th. >n of any neighboring locality. Yet, I repeat, a 



of this question 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 tban myself, who have never given thorough attention to suhjeets of this 

 nature. Hut. whatever may be your decision, 1 can hut congratulate myself that by tin- com- 

 munication of my views relating to observations on Venus, 1 may prove the remote cause of so 

 great anil important an enter] 



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

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

 give glorious result*. 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 (iaiiss, which was copied into my last letter for you, may pi 



of considerable weight. 



1 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 



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

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

 dians of tli- -ive parallels, unless, as intimated, we should succeed in obtaining extra- 

 meridional observations of Venus whilst the sun is above the hoii/on. and finniJtancous measures 

 of the eastern ami western hemispheres maybe combined. Irrespective of the last contingency, 

 I cannot but regard your views directly to the point, and am satislied 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 dilliculty of an important char- 

 acter, but one which did not occur to me until after 1 had despatched my last letter, viz: 

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

 the liori/.oii 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, 

 jamin IVirce, 8. 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 tin- government, should they believe it likely to obtain useful 

 results. Prof. Bachc replied : 



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

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

 Vl --'lly admitted. In discussing the question of the probability of obtaining new data 



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

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

 the two reason* which strongly favor Dr. Gcrling's method are met by any opposing argu- 

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

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

 ineth.-l. Independent methods give the best continuation < >f 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 IMJ of the class used in 



