522 MR JOHN ALLAN BROUN ON THE HORIZONTAL FORCE 
was nearly constant or diminished slowly at St Helena; it then diminished more 
slowly till October, and from November till March more rapidly than at St Helena. 
The movement from March till September1844 resembles that at St Helena. 
TABLE ITX.—MontTuty MEAN VALUES OF HORIZONTAL FORCE (minus A CONSTANT) 


x 
AT St HELENA, 1842-45. 1 00=sp000 
MONTHS. 1842. 1843. 1844, 1845. Means. 
January, : 5 ‘ 66:38 66:98 38°1 23:9 48:84 
February, ; ’ ' 64:86 64:66 [32-3] 27:2 47°25 
March, . , i ‘ 70°44 66:98 27:4 23°9 47-18 
Apuil,, ; «. 4 . : 68°18 66°51 30°8 20°3 46°45 
May, x 5 : ; (2°62 65:05 35'3 20-7 48:39 
June, . : F 3 70:25 65:77 395 229, 49:43 
July, : : . : 63°99 61°54 Sy/ 18-8 45:38 
August, ‘ j ‘ 65°58 56:91 33°2 15:2 42°72 
September, ; i : 63°60 Ray )7/ 31:9 11-0 40°44 
October, : : ; 64:88 52°35 Pray £8) 38°71 
November, h : ‘ 63°60 49-40 25'8 10°4 37°30 
December, 3 4 ; 66:07 44:00 Q7°-4 5:9 35°84 


December 1841 = 57-76, 
22. On the whole, the variations of force at the Cape and St Ilelena confirm 
each other (with some small exceptions) from 1841 till September 1844. This 
general agreement renders it probable that the unusual movements shown at the 
Cape after September 1844, which differ so completely from the movements for 
the same time at St Helena (and at every other place), are chiefly instrumental. 
It has already been pointed out by me* that the results at St Helena may be 
affected by the magnetic character of the whole island. At present, however, I 
feel inclined to believe that any effect due to this cause must be of a very minute 
kind; and I would rather attribute the small differences of the movements 
observable at the Cape and St Helena, in the period before September 1844, to in- 
strumental causes acting at the latter place, if they may not always be explained 
by variations due to the cause of the secular change. 
23. It is obvious, [ think, that even in the first period (before September 1844) 
the movement (continuous increase or diminution) which we attribute to the 
secular cause, does not commence or terminate at exactly the same epochs at 
these two places. Both at the Cape and St Helena (as, indeed, at Toronto) the 
progressive movement appears to be interrupted and inverted by the annual move- 
ment; on this account it must be very difficult to disengage the annual law from 
a variation which has no certain period, and which commences to act or ceases to 
act at different times at places not very distant, such as the Cape and St Helena. 
* Makerstoun Observations 1644, p, 395 (foot-note). 

