xv iii ORIGIN AND OPERATIONS 



The amendment, so reported was duly sanctioned by both houses of Congress ; and on the 3d 

 of August the President approved a law in which was incorporated a direction to the Secretary 

 of the" Navy to expend five thousand dollars, or so much thereof as might be necessary, in 

 making " the observations recommended by the American Philosophical Society and Academy of 

 Arts a'nd Sciences." Preparatory orders for the performance of this service were sent to me 

 shortly afterwards, together with authority to adopt such preliminary measures as I might con- 

 sider expedient. Evidently, conference with the members of the two societies, whose recom- 

 mendations were made the basis of action by Congress, was an essential ; and I proceeded to 

 Philadelphia and Boston, laying my detailed plan of operations before them, and inviting 

 amendments. I quote it here : 



" The purpose for which an appropriation was solicited to send out an expedition to the 

 southern hemisphere being the collection of data which shall add to the accuracy of our know- 

 ledge of the solar parallax, observations to this end will, obviously, demand paramount atten- 

 tion. Two distinct phenomena will occur during the period proposed to be devoted to the sub- 

 ject in Chile, by the investigation of each of which, we may reasonably hope to deduce tolerably 

 correct information of the true parallax, and of the absolute value of the received constant. 

 Taking them in the order of their occurrence, they are: 



"First. The opposition of the planet Mars, commencing towards the close (November 17) of 1849. 

 As an ephemeris of the stars proper for comparison with the planet on each day has been 

 selected at the Nautical Almanac office, it should be scrupulously adhered to by the observers at 

 every station. Two classes of observations of this planet may be made advantageously. 1st. 

 Simultaneous extra-meridional measurements at pre-arranged times, [between the observatory 

 in Chile and that at the Cape of Good Hope, the planet to be about three hours past the me- 

 ridian of the latter, and three hours from the meridian of the former station. The time should 

 be reckoned from the meridian of Greenwich, as better known than Washington; and the lon- 

 gitude of the southern station be ascertained with the greatest precision possible in a preliminary 

 investigation. A comparison of these differential measures will afford a parallax in right 

 ascension ; and as in them the differences of right ascension between Mars and the selected stars 

 are wanted, the diaphragm to the micrometer of the telescope should contain at least five ver- 

 tical wires, that it may be used as a transit instrument. Once directed so that both objects 

 will traverse its field, the telescope should remain undisturbed until the time of transit of each 

 has been recorded for every wire. It is doubtful whether an analogous series can be obtained 

 from the observatory at Paramatta, which lies nearly on the same parallel, because of the 

 planet's great northern declination and proximity to the horizon, when equidistant between the 

 two meridians. But similar series made at the same stations before and after culmination will 

 furnish valuable data, and these may be repeated ad libitum.'] Differential measures of decli- 

 nation should occupy a portion of each night for combination with others to be made at the 

 northern stations, and it is not doubted that the value of these [also] will be increased if made 

 in both hemispheres at the same instant. 2d. Meridional observations. These being chiefly 

 to determine the parallax in declination, in order to guard against errors which may possibly 

 creep into measurements of absolute zenith distance by fluctuations of the circle zero, caused by 

 handling, it is suggested that the following method be adopted, viz: determine the nadir point 

 of the circle by reflection of the horizontal wires from mercury, immediately prior and subsequent 

 to the passage of the first and last comparison stars over the meridian, and, when practicable, give 

 differential measures with the micrometer screw positive preference over changes of altitude in 

 the circle telescope. This last may readily be the case when the declinations do not differ more 

 than 20', as frequently offers in the ephemeris for 1849-'50. One limb of Mars should be 

 observed with the fixed horizontal wire, the other with the nearest micrometer wire; the tran- 

 sits at the I, III, V, and VII wires; and the altitudes, especially, at the II, IV, and VI. The 

 same remarks are applicable to observations proposed to be made at the opposition of the planet 

 in 1H52. 



