352 PHILOSOPHICAL TRANSACTIONS. [aNNO 17 66. 



covery of his health, and the perfect cure he had received. He then went to 

 rejoin his company at Dieppe, and resume his former employments. This ob- 

 servation, at the same time that it furnishes a remarkable instance of animal ve- 

 getation, strongly encourages surgeons to attempt the preservation of limbs, in 

 all cases where there is a possibility of bringing about this sort of regeneration, 

 so useful to mankind, and so honourable to the art. 



XXXIF. A New Method of Determining the Longitude of Places, from Obser- 

 vations of the Eclipses of Jupiter's Satellites. By Mr. fVargentin, F. R. S., 

 and Sec. of the Royal Acad, of Sciences at Stockholm. From the Latin, p. 278. 



M. Wargentin remarks, that the method heretofore practised, to find the lon- 

 gitude of places by means of the eclipses of Jupiter's satellites, especially of the 

 two nearest, which are the fittest for this purpose, is to observe the same eclipse 

 at two different places, noting the exact times, with good telescopes and well 

 regulated clocks; then taking the difterence of these times, that will be the dif- 

 ference in time between the meridians of the two places; or turning that time 

 into motion, at the rate of 15° to an hour, it gives the difference of longitude, 

 in degrees, minutes and seconds, between the two places: then, if the longitude 

 of one place be previously known, that of the other hence becomes known also, 

 viz. by adding or subtracting the observed difference of longitude with that of the 

 given place, according as the case may require. 



But plain and easy as this method appears, Mr. W. observes that it can but 

 seldom be practised, to find the longitudes of unknown places^ on account of the 

 want of good correspondent observations, viz. such as are accurately made, of 

 the very same eclipses, in two places, of one of which the longitude is well 

 known. To remedy this defect then, Mr. W. further observes, that astrono- 

 mers use to follow two ways ; viz. to supply correspondent observations at some 

 fixed observatory, or place of known longitude. Thus, some inquire whether 

 there is noted down, at the known observatory, any near eclipse of the satellite, 

 either next before or after the eclipse in the unknown longitude, to which the 

 correspondent observation is wanted: which being found, then by adding or 

 subtracting the interval of time for the known number of intervening revolu- 

 tions, it gives the time at the observatory of the correspondent eclipse required. 

 But this method is attended with great hazard: for the revolutions of a satellite 

 are unequal, so as that hardly any two successive ones perfectly agree together. 

 It is true that the theory teaches how to compute many inequalities, but yet 

 with much trouble and danger of mistakes. Sometimes also correspondent ob- 

 servations are defective, by their interval of time being too remote from the 

 observed eclipses, or if there be one, out of a great many that is nearer, it may 

 be incorrect or insufficient. 



