VOL. Xril.] PHILOSOPHICAL TRANSACTIONS. 681 



of the satellites happen so frequently, that there are more of them visible in 1 

 year than there are days, though the planet % lie hid under the sun's rays 

 every year a whole month together. 



I know our navigators will object to this method, that it is difficult to prac- 

 tise at sea, because long telescopes are required, which the motion of the ship 

 will not permit them to manage aboard ; that it is hard to distinguish one sa- 

 tellite from another, and that tables or other contrivances for showing their 

 mutual positions are here wanting: to which I answer; That if it be not prac- 

 ticable at sea, they cannot deny but it is at land; that the true longitude of re- 

 mote coasts from us is the first thing desired for the correction of their charts; 

 let them attempt these first, and I doubt not but the success will encourage 

 them so much, that they will readily find means to put it in practice at sea.* 

 That the French have used this method successfully both in Denmark and their 

 own country; that a telescope of 14 feet long at most, or for need one of 8 feet, 

 with broad eye-glasses, will be sufficient for this purpose ; that the difficulty 

 cannot be known till it be tried, and that use renders many things easy which 

 our first thoughts conceived impracticable. 



If it be required to know whether any one of those eclipses invisible to us 

 be visible in any other given place, convert the difference of meridians between 

 it and London into time. And if the place lie to the east of London, add it 

 to, if to the west, subtract it from the time of the appearance at London, the 

 sum or difference accordingly will be the true time of the eclipse under that 

 meridian, at which, if % be above the horizon, and the sun beneath it, the 

 eclipse is there visible, otherwise not. Or by the help of the ephemerides of 

 the planet's places, and a terrestrial globe, the space on it in which any of these 

 eclipses will be visible may be found thus : first seek the true places of the sun 

 and Jupiter, with his latitude, in the ephemerides, whereby you may find their 

 declinations and right ascensions, either by the common tables or the globe it- 

 self exactly enough for this method. Bring London on the globe to the meri- 

 dian, and detaining it there, note what degree of the equator is cut by it; from 

 this subtract the time of the eclipse after noon, converted into degrees and 

 minutes, the remainder shows the longitude of that meridian on the earth, 

 where it is then noon when the satellite is eclipsed, which I therefore call the 

 meridional longitude of the eclipse. Bring this meridional longitude under the 

 meridian, and elevate the nearer pole to the sun as much as is his declination, 

 keep the globe in this position, and if If. be in consequence of the sun, draw 

 a line on the globe along the eastern horizon, it passes over all those places 

 where the sun is setting at that time, but if If be in antecedence of the sun, 



* This, however, has never yet been done. 

 VOL. II. 4 S 



