326
to these Moons of Jupiter may probably be,—What are theirsizes ? This is a query which for some time after their dis-covery was involved in considerable obscurity. Even now wecannot answer it with any pretence to extreme accuracy.From an exact measurement of the ang’les subtended at oureyes by the apparent diameters of their discs, it is no doubttrue, that we can at once calculate the actual sizes, whichat their known distance from us would correspond to suchapparent sizes. But the diameters in question lie between1 and 1 i seconds of angular measure, i.e. they only measurefrom a yj^oth to a x S Wth part of the apparent width of thedisc of the Sun or Moon . They are, in fact, so small, thatmodern instruments of the very best construction and ofthe highest power are required, in order that they may bemeasured with any approach to precision.
The earlier observers were forced to employ other and lessdirect methods ; for instance, Sir William Herscliel, by watch-ing how long the second Satellite occupied in entering uponthe disc of Jupiter , when about to transit across it, obtained avery accurate value for its diameter. He was also remarkablycorrect in his judgment as to the probable sizes of the otherthree. In like manner, Schroter and Harding, by similarobservations, effected a very satisfactory estimate of the dia-meters of all the four Satellites.
Since 1829, however, several careful sets of actual measure-ments have been made. From some of the best of these,combined with such observations as we have previously men-tioned, Professor Engelmann, of Leipzig , deduced in 1871 thefollowing values of the respective diameters, which are mostprobably approximately correct; although it deserves mentionthat measurements made in 1881, by Professor Colbert, withthe great achromatic telescope of the Chicago Observatory,18£ inches in aperture, make them to be considerably larger.
For the 1st about 2,500 miles, or approximately '2i thousands of miles.
2nd „
2,100
2*
fi
3rd „
3,550
31
4th „
2,960
ft
3
It may be interesting to compare the above values with that,