220 



mode of using this apparatus for angular measurements will be seen 

 by the following account of the observations. The observer had 

 also a small transit-instrument by which he obtained true time, using 

 for timepiece his watch, which, as it only indicated minutes, required 

 the supplement of a temporary seconds' pendulum. 



In the account which M. Lescarbault gives of his observations, he 

 says that it had been his practice to examine with the telescope the 

 contour of the sun for a considerable interval on each day in which 

 he had leisure, and that at length, on March 26, 1859, he saw a small 

 round spot near the limb, which he immediately brought to the inter- 

 section of the wires of the telescope. Then, according to his state- 

 ment, he quickly turned the graduated card till two of the wires of 

 the finder were tangents to the sun's limbs, or equidistant from them. 

 But it is evident that to effect an angular measurement in this way, 

 one of the middle wires of the finder must have been placed tangen 

 tially to the sun's limb at the point of their intersection, to which 

 point the spot had just been brought. Assuming that this operation 

 was performed, the angular distance of the point from the vertical 

 diameter of the sun might be read off, as the account states that it 

 was, by applying the plumb-line apparatus to the graduated card. 

 This method could only give a rough measure of the angular position 

 of a point very near the sun's limb ; and in fact M. Lescarbault does 

 not appear to have attempted to determine the position of the spot 

 during the interval between the beginning and the end of the 

 transit. He states that the spot had entered a little way on the sun 

 when he first saw it, and that the time and place of entrance were 

 inferred by estimation. 



The following are the immediate results of the observations : — 

 The spot entered at 4 h 5 m 36 s mean time of Orgeres at the angular 

 distance of 57° 22' from the north point towards the west, and de- 

 parted at 5 h 22 m 44 s , at 85° 45' from the south point towards the 

 west, occupying consequently in its transit l h 17 m 8 s . The length 

 of the chord it described was 9' 14", and its least distance from the 

 sun's centre 15' 22". M. Lescarbault also states that he judged the 

 apparent diameter of the spot to be at most one-fourth of that of 

 Mercury, when seen by him with the same telescope and magnifying 

 power during its transit across the sun on May 8, 1845. The lati- 

 tude of Orgeres is 48° 8' 55", and longitude west of Paris, 2 m 35 s . 



From these data M. Leverrier ascertained, by calculating on the 

 hypothesis of a circular orbit, that the longitude of the ascending 

 node is 12° 59', the inclination 12° 10', the mean distance 0*1427, 

 that of the earth being unity, and the periodic time 19*7 days. Also 

 he found that the greatest elongation of the body from the sun is 8°, 

 the inclination of its orbit to that of Mercury 7°, the real ratio of 

 its diameter to Mercury's 1 to 2 '5 8, and that its volume is one- 

 seventeenth the volume of Mercury on the supposition of equal den- 

 sities. This mass is much too small to account for the perturbation 

 of Mercury's perihelion According to these results, the periods at 

 which transits may be expected are eight days before and after 

 April 2 and October 5, the body being between the earth and sun 



