LETTER FROM LIEUTENANT MAURY, U. S. N. 



OBSERVATORY, WASHINGTON. 



SIR : I have the pleasure to enclose you herewith the observations made at this establishment 

 in connection with your Astronomical Expedition to Chile. They relate to Venus and Mars, 

 and were made with the large equatorial by Mr. Ferguson. The accompanying letter from 

 him to me, descriptive of the method of observing and reducing, contains, it is believed, all the 

 explanations necessary. 



I have not had the force to spare for copying these observations ; I will, therefore, be obliged 

 to you if, when you are done with them, you will return the originals to this office. 



Respectfully, &c., 



M. F. MAURY. 

 Lieut. J. M. GILLISS, 



Astronomer of Expedition to Chile, Washington. 



UNITED STATES NAVAL OBSERVATORY, 



May 1, 1855. 



SIR : I herewith present journals of observations, and reductions of the same, of Mars and 

 Venus, made in conformity with your instructions, and intended to be used in connection with 

 observations of the same planets made in Chile under the direction of Lieutenant James M. 

 Gilliss, for a more accurate determination of the parallax of the Sun. You will remember that 

 early in December, 1849, after Mars had been observed several times and under different atmo- 

 spherical circumstances, I stated to you that, from a discussion of the results of the compari- 

 sons already made, it was very evident that few of them would be available for the purpose 

 intended, as at that season of the year the planet would but seldom be susceptible of sufficient 

 definition to admit of the requisite accuracy of comparison. At the same time, I requested 

 to know from you whether observations should be limited to such times as afforded reason- 

 able hope of available results, or whether you wished me to observe without reference to cir- 

 cumstances. In answer, I was directed to observe whenever the planets were visible this 

 instruction applying equally to Venus and Mars. In consequence, observations have been 

 made, and are now presented, which were not expected at the time to possess the requisite 

 precision, and whose value can only be estimated by the character of the results derived from 

 them. 



The following description of the method of observing ; the state of the instrument ; its 

 adjustment, and the value of the revolution of the micrometer, will supply all the information 

 not given in the record of each observation. 



The observations were made by myself, -without any assistant; the circle of declination 

 being clamped so as to bring the compared objects in the middle of the field, and the polar 

 axis left altogether free ; the position of the instrument being sufficiently assured by its weight. 

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