the Royal Society’s Injlruments . 395 
magnet in the year 1774 was placed nearly in the fame 
fituation and direition that the two were in 1 7 7 5, fo that 
the difference of their effedt in thefe two years can hardly 
have been fo much as 3'; and therefore, the great appa- 
rent alteration of the variation between the two periods 
cannot have been owing to them. Neither can it have 
been owing to the fault of the agate cap ufed in the year 
1774, as the error proceeding from thence could hardly 
be more than 2 or 3'. It is intended that, for the future, 
the abovementioned magnets fhall be kept always in the 
fame fituation and diredlion that they are in at prefent, 
and in which they were in 1775. 
Of the Dipping-needle . 
I N this inftrument the ends of the axis of the needle 
roll on horizontal agate planes, a contrivance being ap- 
plied, by which the needle is at pleafure lifted off from 
the planes, and let down on them again, in fuch manner 
as to be fupported always by the fame points of the axis 
refting on the fame parts of the agate planes ; and the 
motion with which it is let down is very gradual and 
without fhake. The general form of the inftrument, 
the fize and fhape of the needle, and the crofs ufed for 
balancing it, are the fame as in the dipping-needle de- 
fcrihed in Phil. Tranf. vol. LXII. p. 47 6. It is alfo made 
by the fame artift Mr. nairne. 
It may be feen in the Meteorological Journal, that the 
dip was obferved firft with the front of the inftrument 
Vol. LXVI. Ggg 
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