PHYSICAL AND MATHEMATICAL SCIENCES. 243

focus of which the earth is placed. He also threw outsome hints of the parabolic form of the orbits ofcomets.

Another contemporary astronomer, Gabriel Mouton , ofLyons, first practised that important process, the inter-polation, for determining the place of a planet, at someinstant of time intermediate to two others, for whichits place is given in the tables: he also used the pen-dulum to measure differences in right ascension.

The first idea of the transit instrument, a telescopemoveable only in the exact plane of the meridian, towatch the transits of stars across it, though in a veryimperfect form, seems to have been suggested by Roemerabout 1690.

The elder Cassini, was invited from Italy into France by Louis XIV. , in 1669, and settled in the observatory atParis , where he continued a long series of accurate andvaluable labours, particularly directing his attention toperfecting the theory of the interesting system of Jupiter ,which so beautifully represents, on a small scale, thegreater planetary system. He also discovered ultimatelyfour satellites of Saturn , in addition to the one observedby Huyghens .

Both he and Maraldi, and probably other astronomersof the period, observed a highly remarkable circumstanceconnected with the eclipses of the satellites of Jupiter .Their theory had now been sufficiently determined toafford the data for calculating these eclipses; a remark-able difference, however, sometimes appeared betweenthe time of their occurrence, as computed and observed.It was soon found that the emersions took place behindthe calculated time nearly fourteen minutes when theearth was at that part of its orbit furthest from Jupiter ;and that the difference diminished up to the positionw-here it was nearest. It seemed to be connected withthis circumstance alone, and independent of all others.This was, however, only as yet ascertained with certaintywith regard to the first satellite.

r 2