'Notices respecting New Books. 305 



For the determination of the mean distances or major semiaxes of 

 the orbits, measures at or near the greatest elongations can only be 

 advantageously employed. In the course of the observations a good 

 many measures of distance were taken ; but by reason of the want 

 of symmetry of the upper and lower portions of the outline of the 

 visible part of the planet, which renders it extremely difficult to fix 

 upon the position of the apparent centre, they were found (excepting 

 in situations very near the line of the ansa) not only to present dis- 

 cordances among themselves, but to be affected by a general cause 

 of error. The measures of distance are therefore made use of in the 

 case of Titan only, and the resulting value of the semiaxis of the 

 orbit is A=177"-53. 



The author states that he had no confidence in any of the distances 

 of Rhea measured with the equatorial micrometer, the faintness of 

 the object not permitting it to be satisfactorily covered with the wire. 

 They are accordingly not stated. In the case of Iapetus there was 

 less difficulty ; and though the measures were not numerous, and 

 were for the most part taken merely for the purpose of identifying 

 the satellite, it was thought they might be usefully recorded. One 

 of them appears to have been an observation of the greatest elonga- 

 tion, and amounted to 553"'93, which, reduced to the mean distance 

 of Saturn, corresponds to a radius vector of 8' 38"-07. Considering 

 that we have no knowledge of the eccentricity of the orbit, this agrees 

 sufficiently well with the mean elongation (8' 34"*8) calculated from 

 the distance of Titan by comparison of their mean motions. 



Chap. VII. Observations of the Solar Spots. 



"At the latter end of 1836, and during the first half of 1837, the 

 spots on the surface of the sun were extremely remarkable, not only 

 for their number and size, but also in their arrangement and forms. 

 In consequence, during the interval above-mentioned, a great number 

 of drawings were made of the sun's disc by projecting the image 

 formed in the focus of an achromatic finder attached to the equatorial, 

 for which, after a few trials, a perspective day telescope of 20 inches 

 focus, and 1*4 inch aperture, was substituted as more convenient. 

 The image was received on paper pinned on a screen of wood, and 

 traced with pencil with one hand, the other managing the right 

 ascension handle so as to keep the preceding limb of the sun on a 

 fiducial line, previously drawn on the paper, and the centre of some 

 small and well-defined spot on a fiducial dot. When the right 

 ascension motion was allowed to rest, the image of this spot of course 

 travelled away from the dot, and after allowing it to do so till near 

 the edge of the paper, another dot was made, marking its new place, 

 and these two dots being joined by a straight line, gave the direction 

 of the diurnal parallel on the paper. The minuter details necessary 

 to effect a complete resemblance of the projection to the actual ap- 

 pearance of the spots, with their penumbras, &c, were then worked 

 in with the aid of the telescope, by hand, as an eye-draft ; as well 

 as magnified representations of remarkable spots, faculse, and other 

 particulars." 



