398 . l»HlLOSOPHICAL TRANSACTIONS. [aNNO I677. 



meet the light, the revolutions of the immersions will appear to be shortened by 

 so much as those of the emersions had appeared to be lengthened. And be- 

 cause in 424- hours, which this satellite very near takes to make one revolution, 

 the distance between the earth and Jupiter in both the quadratures varies at 

 least 210 diameters of the earth, it follows that if for the account of every dia- 

 meter of the earth there were required a second of time, the light would take 

 3-1- minutes for each of the intervals GF, KL; which would cause near half a 

 quarter of an hour between two revolutions of the first satellite, one observed in 

 FG, and the other in KL; whereas there is not observed any sensible differ- 

 ence. 



Yet it does not follow hence that light requires no time. For after M. 

 Romer had examined the thing more nearly, he found that what was not sensi- 

 ble in two revolutions, became very considerable in many being taken together, 

 and that for example, 40 revolutions observed on the side F, might be sensibly 

 shorter than 40 others observed in any place of the Zodiac where Jupiter may 

 be met with; and that in proportion of 22 for the whole interval of HE, which 

 is the double of the interval that is from the earth to the sun. 



The necessity of this new equation of the retardment of light is established 

 by all the observations that have been made in the Royal Academy, and in the 

 Observatory, for the space of 8 years ; and it has been lately confirmed by the 

 emersion of the first satellite observed at Paris, the Qth of November last, at 

 5 o'clock, 35"^ 45^ at night, 10 minutes later than it was to be expected, by 

 deducing it from those that had been observed in the month of August, when 

 the earth was much nearer to Jupiter : which M. Romer had predicted to the 

 said academy from the beginning of September. 



Of Damps in Mines, By Mr. Roger Moslyn. N° 136, p. 895. 



The coal work at Moslyn, in Flintshire, lies in a large parcel of wood-land, 

 which has a great fall to the sea side, which is directly north. The dipping or 

 fall of the several rocks or quarries of stone that are above the coal, and con- 

 sequently of the coal lying under them, partly crosses the fall of the ground, so 

 that the dipping of it falls within a point or less of due east ; and the stratum 

 runs, in some 40, in others 50, or even 60 yards under the level of the sea. 

 This work is on a coal of 5 yards in thickness, and has been worked about 36 

 or 38 years. When first found it was very full of water, so that it could not be 

 wrought down to the bottom of the coal. But a witchet or cave was driven out 

 in the middle of it on a level, for gaining room to work, and drawing down 

 the spring of water that lies in the coal, to the eye of the pit. In the driving of 

 which witchet, after they had gone a considerable way under ground, and were 



