j v ORIGIN AND OPERATIONS 



indeed, been made, but the planet's place may be found for it by interpolation from the 

 observations at B. By this interpolated observation, let Venus be placed at a point C of the 

 heavens ; then, the celestial arc A C is evidently the measure of the angle under which the 

 known places A and C appear to Venus, and from this angle the horizontal parallax of the 

 planet must be deducible the more advantageously the nearer the terrestrial line approaches 

 to a diameter of the earth. I also imagine that it will be of great advantage if astronomers 

 will unite in making careful meridian observations and promoting micrometrical measurements 

 at the stationary periods to the greatest possible extent. For the latter class of observations, 

 however, it is not only necessary that the times should be previously considered, but also the 

 attending circumstances, and a list of comparing stars be especially selected. No doubt this 

 will soon be done, should astronomers be inclined to adopt my proposition ; but I send you 

 what I have found for 1847, premising that partial examination has also been made for 1849, 

 and that the local stationary point of the latter year oifers much greater facilities to observation 

 than the present. * 



" The preceding synopsis of my paper will, I hope, reach you in print after a while. Mean- 

 time, I beg you will examine the subject, and should you coincide in my views I trust you will 

 interest American astronomers as far as you can, for I flatter myself that observations will be 

 instituted this year at European observatories ; and, indeed, I am sure that a greater number 

 of accurate meridian observations are likely to be made during the months of September, Octo- 

 ber, and November than is common. Besides this, it is more than probable that observatories 

 having the requisite instruments will take advantage of the western stationary epoch to make 

 micrometrical measurements. For the results and success of 1847 it is much to be desired that 

 the few delicate meridian instruments in the southern hemisphere should be brought to co-op- 

 erate with us ; and this, perhaps, it is in your power to facilitate. Of equal consequence will 

 be micrometer observations from the same section of the globe ; but as the latter require no 

 permanent observatory, and only a chronometer, a telescope fitted with a micrometer, and a 

 knowledge of the neighboring stars, such observations may well be made by travellers. Whether 

 there will remain time prior to the eastern period for the necessary instruction of voyagers to 

 the southern hemisphere, I am not able to determine." 



This letter bears date 17th April, but was not received until the early part of July, and the 

 next eastern stationary term was to occur in September. On conference with the late able 

 astronomer, Prof. S. C. Walker, he suggested the immediate publication of the letter, as the 

 mode most expeditious of making it generally known, and, in accordance with his advice, 

 printed copies of a translation were forwarded to all the astronomers and observatories of the 

 United States with as little delay as possible. There was too little time in which to perfect 

 arrangements for more extended co-operation at that conjunction, and Dr. Gerling was shortly 

 notified that the distribution of his letter was probably all that I should be able to do in the 

 work for 1847. But to prove my interest in the prosecution of the problem to its new solution, 

 I then proposed an expedition to Chile, to observe the planet near its stationary terms and 

 opposition in 1849, should my views receive encouragement from astronomers to justify such an 

 undertaking. Nearly on the same meridian as Washington is the island of Chiloe, a place of 

 considerable trade with the nearer ports, and occasionally visited by American whale-ships. 

 At all events, it was accessible without much difficulty, and I hoped to be able to induce the 

 government to send me there, proposing to leave the United States in time to reach the island 

 by the middle of March of that year at latest. To avoid expense, which it was supposed would 

 prove the first and main obstacle, I contemplated only one assistant, who, like myself, would 

 be an officer of the navy, and in the receipt of pay whether abroad or at home, and would take 

 instruments already belonging to. or under control of the government. I proposed Chiloe, 

 because it was the point farthest south on this continent at which a lengthened winter residence 

 could be endured in exposure without incurring an outlay that might prove a serious impedi- 

 ment, and because I thought that a passage to it could be obtained in a whale-ship from one of 



