296 SUNS DISTANCE. 



one of these times, Mars is seen in contact with one star, and at 

 the other time with another star. These observations give the means, 

 as in figure 1, of determining the distance of Mars. And though, in 

 fact, both the Earth and Mars are moving, yet the effects of those 

 motions can be so exactly calculated as to give to the determination 

 the same accuracy as if both Avere at rest. 



In order to compare the value of this method with that of observa- 

 tions on the meridian at two observatories, we must estimate the 

 length of the base line A B in figure 1, or A A' in figure 6. The 

 greatest meridional base line, from Pulkowa to the Cape of Good 

 Hope, is = Earth's radius X 2 sine 47° nearly. The measure of the 

 greatest base line A A' depends on the latitude of the observatory. 

 At Greenwich it is = Earth's radius X 2 sine 38° 30'; at the Cape 

 of Good Hope and at St. Jago (Chili) it is about = Earth's radius X 

 2 sine 57°: at Madras it is nearly = Earth's radius X 2 sine 77°. 

 Thus it appears that at each of the three last-mentioned observatories 

 the base line which can be obtained is considerably greater than 

 the best which can be obtained by meridional combination of two 

 observatories. At Madras the angle to be measured would be about 

 44". To this is to be added that the method is attended with no ex- 

 pense whatever; that the observations which are compared are made 

 with the same telescope and by the same observer, or the same series 

 of observers; that there is none of the tediousness, the wearying cor- 

 respondence, or the doubt, which are inseparable from observations 

 requiring distant co-operation; and that the observer is supported by 

 the feeling that his own unassisted observations will give a perfect sys- 

 tem of means for deciding one of the most important questions in 

 astronomy. The Astronomer Royal expressed his opinion that this 

 method is the best of all. 



In order to use the process to the greatest advantage, Mars ought 

 to be visible at six hours' distance from the meridian on each side, 

 and therefore his declination ought to have the same name as the lati- 

 tude of the observatory. Thus 1860 will be a favorable year for the 

 Cape of Good Hope and St. Jago; 18G2 will be favorable for North 

 Amerfcan and European observatories. It is scarcely necessary to 

 discriminate between them for Madras, where both years are good; 

 1862, however, is preferable to 1860. 



The first equipment for this observation, on the necessity for which 

 special stress must be laid, is an equatorial, firm in right ascension. 

 Many modern equatorials are deficient in this important quality. It 

 would be well in using them to apply a temporary mechanism for fixing 

 the instrument in right ascension, such as its construction may permit. 

 The next, which will be found advantageous, though not strictly ne- 

 cessary, is the apparatus for the American or chronographic method 

 of transits, by which the number of observations may be greatly in- 

 creased, and something will be gained in the accuracy of each. These, 

 with the ordinary clocks and chronometers, <fec, of an observatory, 

 are all that are required. 



The principal rules for the observer would be: To make the obser- 

 vations, as near as practicable, to the six-hour intervals from the 



