and Intensity of the Magnetic Needle. ^68 



equator, will be 5' or 6'. In London, the magnetic colatitude bein^ 

 35' 12', it ought to be about 13' or 13' ; at Port Bowen, 1° 52', or 

 2° 15', all which quantities are very consistent with observations at 

 those places. 



Again, it follows from the hypothesis, that the daily variation ought to 

 be much gi-eater, and the change of daily intensity also gieater in our he- 

 misphere, while the sun has northern declination) than when his declina- 

 tion is south, or than when he is in the equator ; because he will then 

 approach so much nearer the magnetic pole than in the latter cases ; 

 and, in whatever way the influence takes place; we may expect it to be 

 greatest when its action is most direct : this circumstance is also fully 

 confirmed by observation. 



There is, however, one point, and only one that I am aware of, that 

 has the appearance of being opposed to the theory we are examining, 

 and this in candour ought to be stated- It is this, that although we 

 ought to find, as we really do, a greater change in the daily intensities, 

 as the sun advances to the north, yet the mean daily intensity ought to 

 be nearly the same ; whereas, by referring to our tables, it will be 

 found to be constantly decreasing, from the 1st of January, when the 

 experiments began, to the end of April, when the needle was magne- 

 tized, without any such change of temperature, as is sufficient to ac- 

 count for the circumstance. This anomaly, as it is the only one we 

 have met with, leads us to suspect some other cause, and I think it by 

 no means an improbable one, that the daily and hourly use of this needle 

 for four months, might lead to a deterioration of its own magnetic 

 power ; and that some such effect was noticed, seems probable, by the 

 needle being re-magnetised on the 1st of May, particularly as its in- 

 tensity was so much increased by this operation, which could not 

 have happened, had the needle maintained itself in a state of saturation. 



If this was the case, it necessarily prevents us from comparing the 

 intensity of one month with that of another, although the hourly 

 changes will be too small to be aflfected by this cause. I have not at 

 present made any reference to the experiments performed at the Whale 

 Fish Islands, because their number is inconsiderable, and they cannot, 

 therefore, be supposed to have the same weight as the preceding, in a 

 case of this kind. It is, however, satisfactory to find, that they still 

 agree with the hypothesis which has been advanced. If we go through 

 the same calculation here as in the other examples, we find the mag- 

 netic polar distance ML = 14° ; the one of the equator Q / = 68° 43', 

 answering to 4^ 32™, or 7^28"^ a. m., the time when the sun was on 

 the magnetic meridian of Whale Fish Island ; and 1*^ 32"^ p. m., for 

 the time when the sun was at right angles to the same, and when, as we 

 have seen, the daily variation ought to be the greatest westerly. The 

 time of maximum is registered from F^ 10"^ to 1*^ 30™. Again, the 

 amount here (the magnetic polar distance being 14°) ought to be 32' 

 or 38', according to our preceding determination, and the quantity ac- 

 tually observed on one side of the meridian only as 23' ; which, if the 

 night easterly variation had been taken, would certainly have brought 

 the total somewhere about these limits. 



