employed in the Observatory. 221 
bisecting the crosses with the moveable wires of the 
microscopes, and noting the number of revolutions, or 
parts of a revolution, through which the micrometer- 
head has been turned. The interval between the fixed 
and moveable wires thus measured is the deviation of 
the needle from the horizontal position. 
It is necessary to apply a correction for the variations 
in temperature, as in the case of the other Intensity 
Magnetometer, before the true changes in the vertical 
intensity can be deduced. A thermometer inside the 
box that encloses the needle shows the temperature of 
the interior air for that purpose. 
The remaining instrument employed in the magnetical 
observatories in the determination of the before-mentioned 
elements is the Inclination Instrument, more generally 
known as the Dipping Needle. It consists of a gra¬ 
duated vertical circle, in the centre of which is placed a 
well-balanced magnetic needle with a horizontal axis ; its 
length being nearly equal to the diameter of the circle 
in which it is placed, and, consequently, its points, or 
poles nearly touching the inner edge of the circumference. 
The axis of the needle revolves on agate planes; and the 
angle of the circle at which the needle comes to rest from 
the horizontal line shows the amount of inclination or dip 
at any given place. The general law of the inclination 
increasing from the equator to the poles has been before 
alluded to. At Hobart Town, the south end of the 
needle is drawn upwards of 70° from the horizontal 
position by the earth’s attraction. 
It is evident, therefore, if the construction of the 
instrument was perfect, that one determination of that 
angle would be sufficient: but as it is not in the power 
of art to construct a perfect instrument, even by bestow¬ 
ing the utmost skill and care, it. will be necessary to 
point out t imperfections to which this instrument is 
peculiarly 
