OF THE EXPEDITION. XXXV 



tranquil, and generally so clear that measures could be continued long after daylight if the 

 comparing star was so bright as seventh magnitude. Once I commenced when the planet was 

 not more than 3 above the horizon, and, as has been stated in Vol. 1, there were several 

 occasions when the cusps could be distinguished by the unassisted eye ! Its twinkling some 

 mornings was also quite notable ; both the latter being facts in physical astronomy, or rather atmo- 

 spheric astronomy, never yet witnessed from any observatory in the northern hemisphere known - 

 to me. But, favorably as the planet could be seen, and carefully as colored portions were looked 

 for on several occasions, nothing like a spot was ever detected. The meridian observations 

 were prosecuted except whilst the planet was within 5 of the sun, a screen of white cotton 

 cloth completely protecting the circle from the sun's rays. 



When the circle was again in thorough working order, I made a visit to the northern pro- 

 vinces, taking along the magnetical instruments, and determining the elements at five stations, 

 one of them 3,700 feet above the level of the sea. This journey also afforded opportunity to 

 collect many facts respecting the mineral resources and distribution of mines in those provinces, 

 as well as other interesting information not generally known out of Chile. During the month 

 and a half of my absence from Santiago, there was very little work done by the assistants. 

 Clouds had been even more frequent than in the corresponding period of the. preceding year, 

 though rains had neither occurred so often nor continued so long. / 



Autumn proved almost as unfavorable for the zones ; so that from the commencement of 

 summer to the close of this season, we averaged, for the catalogue, only a little more than 800 

 stars per month. July of 1851 was, undoubtedly, the mostunpropitious month of our residence 

 in Chile. During the second series on Mars, comprising 93 days, between 16th December, 

 1851, and 15th March, 1852, I was more fortunate. About 2,000 differential measures were 

 made on seventy-eight, and meridian observations on eighty nights, it having occurred on two 

 occasions that the comparing star could not be seen through the haze hanging over the valley. 

 Generally the atmosphere was so clear that the shades of color of portions of the planet could 

 be perceived with great distinctness and satisfaction, even under quite bright illumination of 

 the field. The following notes appended to the observations of February 26, will show some- 

 thing of the appreciation in which such nights are held. On that night the star of comparison 

 was double; its companion, blue and of the twelfth magnitude, was some 19" south and 6" east. 



"Neither finer night, better images, nor more satisfactory work, since the commencement of 

 the series. The atmosphere is as steady as the earth itself, and so translucent, that not only is 

 the companion seen distinctly under full illumination, but even its blue color is perceptible." 



There were twenty-four measures on that evening, and I am quite sure the place of the planet 

 will be given by them to within less than half a second of arc. In marked contrast are these 

 notes to the observations on the evening of the 1st February. 



" There were two sharp earthquakes, at an interval of 12s. , about &h. 12m. p. M., (nearly twelve 

 hours before,) whose effect seems to have been to change the condition of the atmosphere. At 

 one instant the planet and star are steady, but before half-way across the field are blurred and 

 jumping in a most extraordinary manner. Therefore, the measures could not have been very 

 good, even had there not been the additional difficulty of endeavoring to keep Mars on the fixed 

 wire by means of the tangent screw, whilst the micrometer wire was moved to bisect the star. 

 Thus, only one reading of the micrometer is given, the star being that number of revolutions 

 to the north of the planet's limb." 



Unfavorable as this appears to be, if we divide the thirty observations into four groups for 

 comparison, the change of declination between the mean of the times for the two groups of the 

 north limb differs from the change shown by the south 0".234. Should other observatories 

 have been equally fortunate, a discussion of the observations cannot fail to be of high interest, 

 for it will probably forever decide the possibility of determining accurately the parallax of Mars 

 from meridian or differential measurements. At the close of the series, at least three meridian 



