VOL. LXVIII.j PHILOSOPHICAL TRANSACTIONS. 387 



moon was observed ; © denoting the sun ; * the star ; S Spica Virginis ; R Regulus ; A Aldebaran ; 

 At Atair ; P Pollux ; F Fomalhaut ; Ad Antares. 7th. The extreme difference between the highest 

 and lowest observation, expressed in minutes of a degree ; when the seconds amount to more than 

 30, the next minute above is taken, otherwise the next minute below. 



8di. The latitude in two columns : 1st. The latitude by account, carried on from the land, in the 

 same manner as the longitude by account. '2d. The latitude by observation ; and where the latitude 

 could not be had by observation, it is deduced by account from the last observation, in which case it 

 is included within [ ],. 



9th. The correct longitude from Greenwich deduced from the time-keeper corrected by the sight 

 of lands, of which the longitudes are known, and by observations of the moon, taking a mean of 

 the several observations of the moon made within a short period of each other. The error of the 

 time-keeper, between the longitude corrected by sight of land or observations of the moon, is sup- 

 posed to have arisen by the time-keeper having altered its rate ot going uniformly between tliese ob- 

 servations ; and the intermediate longitudes are determined by the time-keeper on this supposition 

 Where no observations of the time were made, it is deduced by the account from the last observation 

 of the time, and is then included within [ ]. 



10th. The magnetical observations of the variation and dip, in 7 columns. 1st. The variation by 

 azimuth ; '■2d. The variation by the amplitude : * before, denoting the observation to have been in 

 the morning ; * after, denoting the observation to have been in the evening. The variation was ob- 

 served by the officers with the compasses belonging to the ship. 3d. llie dip with the face of the 

 instrument to the east. 4th. Ditto to the west. 5th. The mean dip of the foregoing observations. 

 6th. The mean corrected, or what is supposed to be the true dip. 7tli. The circumstances under 

 which the observations of the dip \\ere made. 



1 1th. The miles run by log. 



The dip was observed with a dipping-needle belonging to the hon. Mr. Cavendish, made by Sisson. 



The following remarks on die dipping-needle and obser\ations are by Mr. Cavendish. 



The ends of the axis of the dipping-needle are made conical, and turn in conical holes of bell- 

 metal, in the manner of Mr. Lorimer's needle, de.scribed in Phil. Trans, vol. 60, p. "9. The dip 

 was constantly observed botli with the face of the instrument to the east and to the west, and the 

 poles were changed twice during the voyage, in order to see whetlier the needle continued well 

 balanced. The use of this metliod of observing is explained in Phil. Trans, vol. (6, p. 396". 



The mean dip corrected is what is supposed to be the true dip. The foundation of this correction 

 is as follows. By the observations on July 12th, when the poles were changed, it appears, that the 

 marked end of the needle was too heavy, so as to make that end point {^ of a degree too low at that 

 place ; and therefore, if we suppose that tlie force of magnetism is equally strong in all parts of the 

 earth, the error thus produced in other places should be to || of a degree as the cosine of the dip to 

 tlie radius. The observations also made when the poles were changed at Suez, agree well enouo^h 

 with this supposition. 



The dip was observed on board the GrenviUe at Deptford, after her return, in the same part of ilie 

 ship in which the observations were usually made, and was found not to ditfer more than 5' from that 

 observed with the same needle in a pretty large garden in London, about ,"' miles distant ; so that the 

 observations on board the Grenville seem to be not much influenced by the iron- work of the ship. 



J[X. An Essay on Pyrometry and Areometry, and on Physical Measures in 

 general. By J. A. De Luc, F. R. S. From the French, p. 41 9. 

 A new hygrometer was the occasion that led Mr. De L. to the present sub- 

 ject. That instrument was made of ivory, as a former was, but in a glass 



3b 2 



