OF THE EXPEDITION. XXV11 



suitable for the ephemerides of Mars and Venus, reducing the catalogue places to the 1st of Jan- 

 uary of the year for which each ephemeris was prepared. As the apparent orhits crossed sev- 

 eral portions of the heavens in which no stars had previously been observed, at my request, 

 both Lieutenant Maury and Professor Bond, at Cambridge, Mass., caused sweeps to be made, to 

 obtain, if possible, suitable comparing stars. These places, together with those of the planets, 

 were plotted on charts, and lithographed copies were sent with the ephemerides to every obser- 

 vatory. The accompanying circular was as follows : 



TO THE FRIENDS OF ASTRONOMICAL SCIENCE. 



In the year 1847 Dr. Gerling, of Marburg, suggested that a new determination of the solar 

 parallax might be obtained by observations upon Venus at and near her stationary periods, 

 provided these observations be made at points far removed from each other. 



In 1847-8 the American Philosophical Society and the Academy of Arts and Sciences recom- 

 mended to the Secretary of the Navy that an astronomical expedition be sent to Chile, for the 

 purpose of making, according to Dr. Gerling's plan, observations upon Venus, in connexion 

 with the National Observatory at Washington. 



By an act of Congress, approved August 3, 1848, the Secretary of the Navy was directed to 

 cause these observations to be made. / 



Being thus authorized by the national legislature, and that nothing which is calculated to 

 impart interest to the undertaking or to give value to its results may be omitted on the part of 

 the American government, I am directed by the Hon. Wm. Ballard Preston, Secretary of the 

 Navy, to announce to the friends of science the objects and plan of the Expedition, and to invite 

 astronomers generally to lend it their co-operation by making, in so far as it may be practi- 

 cable and convenient for them to make, a series of corresponding observations. 



The Expedition has been fitted out on a scale commensurate with the objects in view. All 

 the means and facilities for it which Congress has placed at the disposal of the Executive have 

 been afforded to it by the Secretary of the Navy. Reposing special trust and confidence in the 

 zeal and ability of Lieut. J. M. Gilliss, U. S. N., he has appointed that officer to the charge of 

 it ; other officers of the navy have been detailed to accompany it as assistants. Passed Mid- 

 shipmen A. MacRae and Henry C. Hunter, who are to accompany it, have been stationed at the 

 National Observatory for the requisite and previous training. The necessary instruments have 

 been procured for the Expedition, and suitable buildings to serve as an observatory in Chile 

 have been prepared in Washington. They are wooden structures, and will be taken to pieces 

 and shipped to Valparaiso in the course of a few days. 



The principal instruments which the Expedition will carry with it are two telescopes equa- 

 torially mounted, a meridian circle, a clock, and three chronometers. 



The larger telescope is an eight and a half feet refractor. It has an object-glass by Fitz, of 

 New York, that affords a clear aperture of six inches and a half. It is fitted with clock-work 

 by Wm. Young, of Philadelphia, and by him provided with a micrometer adapted both for dif- 

 ferential measurements and for measurements of angle of position and distance. 



The other telescope is a five-feet achromatic, by Frauenhofer. It, also, has been equa- 

 torially mounted and fitted with a micrometer by Young, of Philadelphia. 



The meridian circle is by Pistor & Martins. The object-glass of the telescope has an aper- 

 ture (clear) of four and one third inches, with a focal length of six feet. The circles are thirty- 

 six inches diameter, minutely divided, and provided each with two reading microscopes. 



The series of astronomical observations, in which the co-operation of other observers is more 

 especially invited, will consist of differential measurements during certain portions of the years 

 1849, '50, '51, and '52 upon Venus and Mars, with certain stars along their paths. 



The observations upon Venus which will most command the attention of the Expedition, will 

 be differential measurements upon that planet in the morning and evening while it is near the 

 inferior conjunctions of 1850 and 1852. 



