504 MR JOHN ALLAN BROUN ON THE HORIZONTAL FORCE 
APPENDIX. 
Determinations of Corrections employed in the preceding Paper on account of Turn- 
ing the Torsion Circle, or Accidental Causesof Change of Scale-Reading ; with 
the Connection of Series before and after Readjustment. 
105. Maxerstoun.*—During disturbances, when the scale went out of the 
field of the reading telescope, the torsion circle arms were turned till the magnet 
was again forced into the field: it was necessary, after the disturbance had ter- 
minated, to return the arms of the torsion circle to the original reading, when it 
was supposed that the scale-reading for the same force would be as before the 
disturbance. Experiments made in 1842, by turning the arms of the torsion 
circle backwards and forwards, and then returning them to the original reading, 
supported this supposition. A more extensive induction, however, has shown 
that the smallest shocks, and the most careful manipulations, are frequently ac- 
companied by change of instrumental readings. 
106. The occasions (after 1841) on which the arms of the torsion circle were 
moved, and afterwards returned to their former positions, were—1842, July 2; 
1843, May 6; 1844, November 16; 1845, April 13 and December 3. 
The Makerstoun bifilar was also readjusted after 1841; 1843, April 28 and 
November 10; 1846, January 1. 
On the latter occasion the arms of the torsion circle were simply moved 
through a small angle without returning them afterwards. In the case of the 
readjustments, the series of observations after adjustment were connected with 
the series before, by assuming that the mean force for three days before adjust- 
ment was equal to the mean force for the three days following it. This supposi- 
tion may manifestly be inaccurate. In the investigations which have occupied 
the preceding pages, it was necessary to correct the errors of these hypotheses; and 
it will be evident from these investigations themselves, that the errors may be de- 
termined within small limits by comparing the daily means at the place in question 
with those for other places, where the secular change does not differ much, or the 
interval is small. This method has been adopted throughout (taking care to avoid 
days of great disturbance) in determining the true changes of force at any place 
where an instrumental adjustment or accident has rendered this necessary. 
107. The series of observations at Makerstoun in 1842 was too limited (con- 
* Lat. 55° 34’ 45” N., Long. 0° 10" 3-5° W. Height above the sea, 218 feet. Observations 
Trans. Royal Soc. Edin., vols, xvii., xviii,, xix. 

