SUN S DISTANCE. 



297 



meridian on both sides, and to repeat the observations in continued 

 sequence morning and evening, morning and evening. If different 

 observers are employed, to take care that each observer is charged 

 as often with morning as with evening observations. To determine 

 the difference between the right ascension of Mars and the right 

 ascensions of two stars, one having greater N. P. D. and the other 

 smaller N. P. D. than Mars. To use the same stars in at least two 

 observations of different names, morning and evening, and in as many- 

 more consecutive observations as can be conveniently arranged. 

 When it becomes necessary to change the selection of stars, to ob- 

 serve both the old pair and the new pair in one morning or evening 

 observation. In all cases to observe, by such alternation as is most 

 agreeable to the observer, both limbs of Mars, (the preceding and 

 the following.) The observations might with advantage commence a 

 fortnight before opposition and terminate a fortnight after it. 



In the nature of external preparation, applying generally to all 

 observatories, the principal requisite is a chart of the apparent path 

 of Mars in considerable detail, giving the place of the planet for 

 every hour or every few hours, and giving the places of all the stars, 

 little and great, in its neighborhood. The observer in possession of 

 this will be able to select stars of such a magnitude as he judges most 

 agreeable to his eye, and at such intervals as will be convenient for 

 his system of wires; and to attend rigorously to the condition of 

 always comparing the planet with two stars, one of greater and one 

 of less N. P. D. It might be proper that the color of the stars should 

 be noted, in order that, to avoid possible inequalities of refraction, 

 stars of the same color as Mars (if there are such) may be selected. 

 It would not, perhaps, be too much to expect such charts for 18G0 

 and 1862 from the superintendents of our national ephemerides. 



On reviewing the whole subject, the Astronomer Royal presses on 

 the attention of astronomers the importance of observing Mars in 

 1860 and 1862; and for this purpose the necessity of speedily making 

 the preparations, instrumental and literary, which he has described, 

 especially that of the charts of stars with the path of Mars. .At the 

 same time he urges that the future astronomical public will not be 

 satisfied unless all practical use is made of the transits of Venus of 

 1874 and 1882; and that for these a thorough discussion of the ele- 

 ments of the orbit of Venus, the determination of some distant longi- 

 tudes, and a reconnoissance of Wilkes's land must be effected within 

 a few years. 



