OF THE EXPEDITION. XIX 



' ' Second. Observations of Venus about the times of the inferior conjunctions with the sun, and more 

 particularly near its stationary terms. The earliest series of which it will be practicable to take 

 advantage will commence in the autumn (spring of Chile) of 1850; the latest that I contemplate 

 observing will occur in the summer (winter) of 1852. And thus the results of two returns of 

 each phenomenon will be brought to bear on the problem of parallax. As in the case of Mars, 

 the observations are divisible into two classes meridional and extra-meridional ; though, from 

 the vicinity of the planet to the sun, the results derivable from the latter do not appear to be 

 regarded with the same degree of confidence. I propose to commence the principal series, and 

 to continue it as long as the horizontal parallax amounts to 15" that is, about 110 days; and 

 to this end will prepare a map of that portion of the heavens in which the apparent orbit of 

 Venus lies, projecting thereon all stars within 30' of the path which are contained in published 

 catalogues. The map, and a list of stars to be selected from it that will best serve for daily 

 micrometrical comparison, will be sent to every observatory to facilitate the observers' acquaint- 

 ance witli the heavens, and insure certainty of comparison with the same star. Similar remarks 

 to those respecting the extra-meridional observations of Mars apply here also; if any such be 

 found near the orbit, a bright star will serve for simultaneous pre-arranged comparisons during 

 daylight with observatories lying near the same parallel for a parallax in right ascension. Dr. 

 (k-rling also advises the comparison of differential extra-meridional observations made under 

 one meridian, with meridian observations made under another in the opposite hemisphere, the 

 motion of the planet in the intervals of time to be derived from the Nautical Almanac by inter- 

 polation. It is certainly proper that his opinion be received with great deference. 2d. When 

 possible, the meridian measures may and should be made differential at all the observatories, by 

 invariably referring to the nearest bright star as a zero point of the heavens. If this be done, 

 the value of the observations will probably be much enhanced. 



"As the preceding series will occupy only a portion of the time that it is intended to pass in 

 Chile, there may be various shades of opinion respecting the subjects of investigation most 

 desirable, or most likely to produce valuable scientific fruits, and with which we shall employ 

 ourselves at intermediate intervals: but it will probably be admitted by all, that the following 

 cannot fail to be useful if the observations are successfully made. 



" 1st. Meridian observations of the moon, both in declination and right ascension ; the former 

 with a view, more particularly, to the improvement of the constant of lunar parallax, and the 

 latter, in combination with the culminating stars, for the longitude of the station. To verify 

 Burckhardt's semi-diameter, both limbs will be observed whenever practicable, out of and on 

 the meridian, at opposition, and near conjunction. The limited assistance asked for will not 

 permit these observations to be made when the moon passes the meridian later than 1 A. M., 

 except during the oppositions of Mars and western stationary terms of Venus. 



" 2d. Meridian observations of the smaller planets when they culminate within one hour of any 

 observation previously enumerated, and not later than 1 A. M. 



"3d. Lunar occupations that occur before 1 A. M., except such as may be visible to the north- 

 ern observatories also. These will be observed whatever the hour. 



"4th. A catalogue of the stars between the south' pole and 30 of south declination to the 

 eighth magnitude, inclusive. I propose to devote at least three hours of every clear night to 

 this work, and to obtain not less than three observations of each star, every zone to embrace as 

 many previously observed stars as possible. The latter will serve as points of reference. It is 

 not intended to rely wholly on these, but to ascertain the nadir point of the circle before and 

 after each series, and clamp the instrument to the altitude of the centre of the belt to be exam- 

 ined, using only the micrometer screw for determining differences of declination. Supposing 

 that a zone of 40' in declination by three hours in right ascension may be swept every favorable 

 night, if we allow a loss of one third for cloudy weather and overlapping, it will still be pos- 

 sible to sweep the 00 in three years. 



" 5th. It has been suggested that much information respecting terrestrial refractions would be 



