OF THE EARTH’S MAGNETISM. 535 
55. These quantities, by the equation of sines, give (#=0, January 16) 
X= 4-4921 +0-0035 sin (9+ 261°) + 00018 sin (26 + 111°). (8) 
Considering again the last term (which we may suppose as before to some extent 
independent of error of corrections) in units of aaa we have 
2°84sin(26+111°), . : : : ‘ : : ‘ : (9) 
Comparing this with the mean result for Makerstoun and Hobarton, or 
ZU REN ELSA DEB yon Raorhark Soke dt attain nat ik bie eh gt aT (0) 
we find nearly the same coefficient, and the same epochs for the double maxi- 
mum and minimum within nine days. 
56. Annual Period from Observations of Absolute Intensity, Hobarton.—There 
remain now only the observations at Hobarton to discuss. These observations 
were made during the five years 1846-50; and the result obtained by General 
SaBINE is just the reverse (as he has pointed out*) of that he obtained from 
the Cape. 
57. Only one observation was made monthly in 1846 and till April 1847; from 
April 1847 till October 1848, three observations were made monthly ; from 
November till the end of the series, one observation was made weekly. From 
the commencement till October 1848, the observations were made at the begin- 
ning of each month; and as the observations were reduced to the mean bifilar for 
the month, or that corresponding to the middle of the month, there will generally 
be an error on account of the difference of bifilar temperature for the two epochs, 
and the error of the temperature coefficient employed. I have attempted to 
correct this error, but the data are not sufficient, and there appear to be accidental 
errors, which could only be corrected by reference to the original observations.+ 
Thinking it possible that the accidental errors might destroy each other, I have 
taken the results as they are in vol. il. (Introduction, pp. 39, 40). In order that 
the value of the determinations for each month might depend on the number of 
observations made, I have given the change of the force from any one month to 
the next following, a value equal to the sum of the numbers of deflections made 
* Cape Observations, vol. i., p. lxxi. 
+ The bifilar temperature is not given with the observations of absolute intensity, In attempt- 
ing to determine this temperature by the date and the ordinary hourly observations, it appears to me 
that there are several errors in the tables. Thus, as to the dates 1847, March 24 08, August 14 0b, 
September 54 0", October 34 04, &c. &c., of Hobarton time, they are all noon of Sunday, when no 
observations were made; the days are perhaps Gottingen time, and the hours Hobarton time? Again, 
the bifilar readings given as those during the observations 1846, April 84 12; 1847, February 240", 
March 12 0, April 1¢ 08, December 34 0", December 44 04, &c., appear to be erroneous; in some 
cases, perhaps, the error is one of 10 divisions. 
VOL. XXII. PART It. 6Y 
