408 
Annual Period. 
6th, The annual period confirmed by each year’s observation, and 
by groups of years at Makerstoun and Hobarton, at Trevandrum, 
Singapore, Toronto, &c., is, that the horizontal intensity of the 
earth’s magnetism is a maximum near the solstice and a minimum 
near the equinoxes. The equations for Makerstoun and Hobarton, 
according to the function of sines, for the group of years July 1844 
to June 1848, are as follows: — 
Makerstoun, y= 5*20 + 1-66 sin. (0 + 161° 47') + 2*98 sin. (20 + 
129° 34') + 0-80 sin. (30 + 31° 42') 
Hobarton, • y = 4*90 + l*64 sin. (0 + 217° 38') + 2*54 sin. (20 + 
129° 1 T) + 0-45 sin. (30 + 172° 46') 
Where 0 = 0 July 16, and the unit is one 100,000th of the hori- 
zontal intensity at each place. 
7 thy The epochs for the total curve deduced from these equations 
Principal Minimum 
„ Maximum, 
„ Minimum, 
,, Maximum, 
Makerstoun. 
October 1. 
December 22. 
March 6 5. 
June 30*5. 
Hobarton. 
September 16. 
January 1. 
April 4. 
June 17. 
8th, The law is confirmed by the observations at Munich in 
1843-5, made with a unifilar instrument where deflecting magnets 
are used, and where the probable error due to the varying torsion of 
the silk suspension-fibres, and the small directive force of the sus- 
pended magnet, renders such a confirmation the more remarkable. 
9th , The law is also confirmed by a discussion of observations of 
absolute horizontal intensity made at Toronto, the Cape of Good 
Hope, and Hobarton. The results of these observations are shown 
to be faulty, from the inaccuracy of the temperature coefficients 
employed ; but the error due to this cause will appear in the term A 1 
sin (0 + C t ) of the equation of sines, while the term A 2 sin (2 0 + C 2 ) 
on which the double maximum and minimum depend, may be ex- 
pected to be free from the error, and to show the law. The obser- 
vations are discussed, and the term obtained in each case differs 
little from the mean of the terms from the Makerstoun and Hobarton 
bifilars, or — - 
