TOL. XLVIII.] PHILOSOPHICAL TRANSACTIONS. 46I 



short chopping sea, and sailing obliquely across the waves, the plate would fre- 

 quently be drawn from one wave to another through the air, without touching 

 the water; and would jump from one wave to another; the unevenness of the 

 surface, joined to the quickness of the motion, not permitting the plate to follow 

 the depression of the water. This evil he endeavoured to remedy, by placing on 

 the line, at a small distance before the plate, some hollow bullets, such as are 

 made for nets, in order to keep the plate so low down in the water, as to be 

 below the bottom of the waves. This, in part, he found they did; but at the 

 same time they added so much resistance, in their passing through the water, 

 that the inconvenience was as great one way as the other. 



On making up the account of this run, the number of rotations were less, by 

 full one-third, than they ought to have been, compared with former observations, 

 which afforded a convincing proof, that this instrument was considerably retarded 

 in quick motions. The length of the line made use of was about 20 fathoms, 

 which he found necessary, that the water, disturbed by the body of the ship, 

 might be tolerably settled before the plate was drawn through it ; but this length 

 of line was also an inconvenience, as it met with greater resistance in the water. 



On the whole, it seemed that such an instrument is capable of measuring the 

 way of a ship at sea, when its velocity does not exceed 5 sea miles an hour, to a 

 degree of exactness exceeding the log. It therefore may be useful in the mensu- 

 ration of the velocities of tides, currents, &c. and also in measuring distances 

 at sea in taking surveys of coasts, harbours, &c. Thus far it seems capable of 

 performing, on the supposition that it cannot be brought to a greater degree of 

 perfection. But this he was very far from supposing: on the contrary, he thinks 

 that it may be brought to answer the end of measuring the way of a ship at sea 

 universally. 



LXXI. Observations of some Eclipses of Jupiter s Satellites at Lisbon in 1753. 



By J. Chevalier, p. 546. 



LXXII. Observation of a Solar Eclipse at Lisbon, Oct.^Q, 1753. By J. 



Chevalier, p. 546. 



An account of the above two articles is contained in the following. 



LXXIIL An Account of some Astronomical Observations taken at Lisbon by M. 

 J. Chevalier in the Year 1753. By J. Short, M.A., F.R.S. p. 548. 



This gentleman mentions two emersions of the satellites of Jupiter, viz. one 

 of the first, and another of the third, both observed, in a very clear air, with a 

 Gregorian telescope 6 feet long. Dr. Bevis, from a great number of observations, 

 has computed formulae of tables for the times of the immersions and emersions 



