5Q4 PHILOSOPHICAL TRANSACTIONS. [aNNO J 775. 



glass lifts off occasionally. The whole box turns round on a strong brass centre, 

 fixed in a double plate of mahogany, glewed together cross-ways, to prevent its 

 warping or splitting ; and this again is supported by 3 brass feet, such as are 

 used for the cases of table knives, frosted that they may not easily slip, if the 

 vessel should have any considerable motion. It has another square deal box to 

 lock it up in, to preserve the glass, &c. when it is not wanted for use. 



The use of this instrument is very plain, as the inclination or dip is at any time 

 apparent from inspection only, and also the variation, when the frame is turned 

 round till the great vertical circle lies exactly in the plane of the true meridian : 

 for the circle of inclination being always in the needle's vertical plane, its edge 

 will evidently point out on the horizon, the variation e. or w. But at sea, when 

 there is not too much motion, the frame is turned round, till the vertical circle 

 be in the plane of the sun's rays ; that is, till the shadow of the one side of it 

 just covers the other, and the edge of the circle of inclination will then give the 

 magnetic amplitude, when the sun is rising or setting ; but the azimuth at all 

 other times of the day ; and the true amplitude or azimuth being found in 

 the usual way, the difference is the variation. When the motion is consider- 

 able, observe the extremes of the vibration, and take the mean for the magnetic 

 amplitude or azimuth. When the sun does not shine so bright as to give a 

 shadow, set the brass circle in a line with his body, if he be at all visible by the 

 eye. The principal advantage at first aimed at in this compass, was to contrive 

 a dipping-needle, which should be sufficient for making observations at sea. As 

 those needles, to be of use, must be placed, by some means or other, in such a 

 manner, as that all their vibrations shall be made in the true magnetic meridian, 

 north and south, otherwise they are good for nothing. For if one of them be 

 placed at right angles, across the magnetic line, it will stand perpendicularly up 

 and down in any part of the world ; the least dip therefore is always in this mag- 

 netic line. But the only method of setting a dipping-needle at sea, was to place 

 it in a line with the common compass needle ; and this must be very inaccurate, 

 if they be at any considerable distance from each other ; or if they be near, the 

 2 needles would influence each other, and neither of them could be true ; nay, 

 supposing them for once to be properly placed in this line, the least motion of 

 the ship throws them out again. But this instrument has a constant power in 

 itself, not only of setting itself in the proper position, but also of keeping itself 

 so ; or of restoring itself to the same situation, if at any time it has lost it ; and 

 it is curious to see how, by its double motion, it counteracts, as it vi^ere, the 

 rolling motion of the vessel. 



Fill. Bill of Mortality for Chester for the Year 1773. By J. Hay garth, 



M. D., F. R. S. p. 85. 



