Dr. Wentworta Erck on the Satellites of Mars. 31 
in their day. One of them was presented by the,then Duke of Northumberland 
to Cambridge Observatory, where it still is. Another was purchased by the late 
Mr. Edward Cooper, of Markree Castle, in the county of Sligo, where it still 
remains. The third, and best of the three, became the property of the late Sir 
James South, by whom it was presented to Trinity College, Dublin, in whose cbser- 
vatory at Dunsink it now is. 
None of these telescopes, however, were up to the required mark. 
From the death of Fraunhofer in 1830 till, perhaps, 1845, the only maker of 
repute was Mertz, of Munich, who constructed nearly all the great telescopes 
within these dates. Some of his object-glasses reached a diameter of even 15 
inches. 
But about the latter date two other eminent makers of telescopes appeared, viz., 
Cooke, of York, and Alvan Clarke, of Boston, U.S., who have produced the finest 
and the largest telescopes yet constructed. The former having completed, in 1873, 
the great refractor, at Gateshead, in the County of Durham, 25 inches in diameter ; 
and the latter having, very shortly afterwards, produced the great Washington 
refractor, 26 inches in diameter. This refractor, regard being had to both its 
quality and its size, is undoubtedly the finest in existence, but it will not long en- 
joy this pre-eminence, as we hope it will soon be equalled in quality and exceeded 
in size by the Vienna refractor, of 27 inches aperture, now in course of construction 
by Mr. Grubb, of Dublin, who, during the last few years has supplied most of the 
great telescopes, both reflectors and refractors, in.Europe, America, and Australia. 
Thus it appears that prior to 1846 there were no telescopes capable of showing 
the satellites’; and since 1846 there have been no oppositions of Mars nearly so 
favourable for viewing the satellites as the. recent one, so that we may say, with 
almost certainty, it was impossible they could have been discovered earlier than 
they have been. 
Detailed elements of the satellites having appeared in several of the scientific 
papers, it is unnecessary to repeat them here; but there is one circumstance con- 
nected with the inner satellite which is deserving of notice as it is entirely without 
precedent in the Solar System—it is this: that the angular motion of the satellite 
in its orbit is more than three times as rapid as that of its primary round his axis, 
that is to say, while Mars rotates once the satellite completes three and a quarter 
revolutions round him. There is no other known instance in which a satellite 
completes its orbital revolution in less time than the primary completes a revolution 
on his axis. 
The next swiftest moving satellite, in reference to the rotation of its primary, is 
satellite I of Saturn, which completes a revolution in orbit in 2.2 rotations of 
Saturn himself. 
I shall now proceed to my own observations of the outer satellite. On 
the 2nd September I, for the first time, looked for it, and at once perceived 
F 2 
