560 MR JOHN ALLAN BROUN ON THE HORIZONTAL FORCE 
and the corresponding values after turning the circle-arms are found by the 
formula 
(N’— 45-0) 0:000217, 
where N and N’ are the scale readings before and after the change corrected to 
54°-0 Fahr. These formule have been employed. 
124. It has to be remarked, that about 1:5 per cent. of hourly observations 
were omitted at Hobarton; and the daily means given in the Hobarton volumes 
were obtained simply by dividing the sum of the scale readings at the hours of 
observation in each day by the number of observations; thus, if only 22 hours 
(out of 24) were observed, the sum was divided by 22. It is obvious that this 
method must frequently cause error in the means, since, if the observations 
omitted were near the time of a minimum, the mean would be too low; and if 
near the time of a maximum, the mean would be too high. To avoid these 
errors as far as possible, all the observations from 1841 till 1848, which were 
omitted, have had their places supplied by quantities interpolated from preceding 
and succeeding observations, and the means have been taken anew.* 
125. TrREvaANDRUM.}—In correcting the observations for the preceding paper, 
the value of the temperature coefficient, 
q=1-98 se. div. (k=0-000144), 
was employed. This result was obtained from the observations for 1844 alone, and 
when these observations contained some errors that were afterwards discovered. 
As the changes of daily mean temperature at Trevandrum are generally less than 
1° Fahr., the conclusions of this paper will be unaffected by this error, and it has 
not been thought desirable to repeat the projections of daily means, or to recom- 
pute the four weekly means; but the daily means, Table XXXIII., have been 
recorrected by the true value of g’=2°17 sc. div. (See Bifilar Magnetometer, 
Trans. Royal Soc. Ed., vol. xxii. p. 484). The following corrections were applied 
to the observations at Trevandrum :— 
126. 1842, January.—tThe first day after the adjustment showed a consider- 
able increase, probably due to the wires; ten divisions were subtracted on this 
account from January 2 to 7. The means for January 9 and 10 were interpolated 
from Singapore} and St Helena. 
127. 1842, April 3° to 11°, both inclusive. The telescope was moving; the 
differences from April 1° at Singapore were substituted instead, and 4:3 scale 
divisions have been subtracted from all the means before April 1°. This cor- 
rection was found as follows :— 
* The error here noted is much more marked in the case of the hourly means for each month, where 
the same method of summation and division is followed. With some classes of observations (as those 
of the barometer and thermometer), the hourly means are scarcely comparable through these omissions. 
+ Lat. 8° 30’ 32” N., Long. 55 7m 59s E. Height above the sea, 200 feet. Observatory of his 
Highness theRasau of Travancore; Joun Catpecort, F.R.S., Director 1838-1849. The observa- 
tions in manuscript are in the archives of the Royal Society of London. 
{ The agreement between the Trevandrum and Singapore means is so considerable, that for all 
ordinary purposes, a few days wanting in the one may always be supplied from the other without any 
risk of error. 

