THE HONGKONG GOVERNMENT GAZETTE, 29TH MAY, 1886.
469
The mean of the eight readings and the corresponding four times indicated by the chronometer, whose error is known, are respectively taken. The azimuth of Polaris : A, counted from true north, cor- responding to the mean of the times is calculated by the equations :-
tan M-tan 8 sec t
tan A=cos M tan t cosec (☀—M)
where 8 is the declination, t the hour angle of Polaris, and the latitude.
On the determination of the temperature correction.
י.
The deflecting magnet was fixed in the middle of a water-tight wooden box placed on a bar fixed like the deflection bar, and the magnetometer was revolved till the two magnets were at right angles to each other.
The difference The magnetometer was then clamped and the circle read off. between this reading (magnet placed) and the reading before the deflector was placed (magnet away) being Water at the highest of the three temperatures chosen was now poured into the box, and As soon as the a thermometer, whose error had been previously determined, moved about therein.
Water temperature, t, became constant, the scale was read, the angle of deflection being now -u.. at the next temperature, te, was then substituted, and the corresponding angle of deflection, u,, deter- mined. Water at the lowest temperature, t1, was then substituted, the corresponding deflection being The magnet, whose ends were of course left open, was found to assume the temperature of the water as soon as this became uniform.
1.
The temperature co-efficients q and q' were then calculated from Balfour Stewart's equations (Comp. G. M. WHIPPLE: "On the Temperature-correction and Induction-coefficients of Magnets," in proceedings of the Royal Society, No. 181, 1877):-
x + X (ts-t) y (tat,)
x (tt) + y (t-t)
q x cosec u
sin u,- sin u sin u,- sin u q'=y cosec u
The operation was repeated and the readings corrected for change of declination. The correction being obtained in the form: q (t − t1) + q′ (t—t1) suitable for reducing the magnetic moment to t1 degrees was converted to the form: qt+q't, for reducing to 0°, by aid of the corrections corresponding to 0°, 20° and 40° calculated from the former formula.
Specimen of observations; 11th January, 1886.
Magnet away (at commencement),....
"?
19
placed,. away (at end),
..46 2 5
...64
59
20
..64° 58′ 35′′ | u=18° 56′ 30′′
Decl. increasing. 45" in one hour.
Temp.
Temp.
-Scale d.
Fahr.
Scale
Fahr.
d.
O
204.8
94.8
205.5
95.4
204.7
95.2
205.4
94.9
204.75
95.1
205.2
94.7
u-189 U-18 50 U-18 43
56′
30′′ at 5°.0 Cent.
30 at 20°.0 30 at 35°.0
27
}}
197.8
67.8
198.0
67.6
197.9
67.9
198.4
67.8
197.8
68.1
198.4
68.2
197.9
68.2
198.3
68.4
191.1
40.5
192.4
40.6
191.8
41.2
192.4
40.9
191.85
41.4
192.5
41.3
Reduction to 5°.0: + ·000 305 9 (t−5°) + '000 002 06 (t−5°)'
to 0°.0: + 000 285 t + ·000 002 07 t2
On the determination of the Induction-coefficient.
The induction coefficient is, after Lamont's method, determined from deflections, the magnet being placed with its N-pole alternately upwards and downwards at the same distance from the suspended needle. But as the vertical force is small in Hongkong, the apparatus was arranged for also observing horizontal induction, the magnet being then placed with its N-pole alternately northwards and south- wards.
Let m be the magnet moment of the magnet
X
""
19
i
>"
""
15
φ'
"
>>
horizontal force.
dip.
deflection, N-pole of magnet respectively downwards or northwards. deflection, N-pole of magnet respectively upwards or southwards..