A. MATHEMATICS AND ASTRONOMY. 21 



These experiments showed that a displacement could be 

 produced on either side of the line of comparison by varying 

 the method of observation, and Secchi comes to the conclu- 

 sion that such systematic errors may appear without the 

 observer having any sure means of detecting their existence. 

 He points out that the agreement of a long series of such 

 observations is no sure test of the star's motion, since the ob- 

 server always places the instrument in the position where 

 the vision is best. Further researches on this point are then 

 most important. 6 JB, A2yril 3, 1876, 761. 



DIMENSIONS OF THE SATELLITES OF JUPITEE. 



Taking a mean of the measures of the diameters of the 

 satellites ofJiipiter, by Struve at Dorpat and Engelmann at 

 Leipzig, we have for the apparent diameters of the four sat- 

 ellites at the mean distance of the primary 



I. l.OiS"; 11. 0.911"; III. 1.513"; IV. 1.278". 



With a solar parallax of 8.875", the true diameters in En- 

 glish miles will be 



I. 2435; II. 2115; III. 3514; IV. 2970. 



The angular diameters at the centre of Jupiter are 

 1.31.4'; II. 17.1'; III. 17.8'; IV. 8.G'; 

 and the mean distance from J'u2nte7'''s centre 



I. 266,700 miles. 



II. 424,300 " 



12 A, Mb. 10, 1876, 288. 



III. 676,800 miles. 



IV. 1,190,400 " 



THE SATELLITES OF SATUEN. 



The 26 - inch Clark equatorial of the Naval Observato- 

 ry of Washington has been employed during the past op- 

 position of Saturn in observations of his eight satellites. 

 The results of five months' observations by Professor Hall 

 are published in the Astrono7nische JSFachrichten, No. 2076, 

 and are very complete ; especial attention has been paid to 

 Japetiis and Titan^ with the object of determining tlie mass 

 of Saturn. Hyperion (the faint satellite discovered by G. 

 P. Bond) has been observed no less tlian forty times in 

 this single opposition an unprecedented feat, as even Las- 

 sell, in 1864, secured ten measures of position -angle alone. 



