686 MR J. A. BROUN ON THE DIURNAL VARIATION OF THE 



Change of Epochs of Maxima and Minima. 



In order to study the change of epochs of the minima and maxima of the 

 ranges of the mean diurnal variation of magnetic declination at each station, we 

 may divide the stations into two groups — the extra tropical and the intertropical. 

 It will be at once evident, that the epochs of minima and maxima for the northern 

 groups have the same, or nearly the same, position relatively to the December and 

 June solstices, that those for the southern groups have to the June and December 

 solstices. 



Thus Makerstoun, Toronto, Hobarton, and the Cape, have only one minimum 

 and one maximum of range. At Makerstoun they occur near the December and 

 June solstices ; at Hobarton near the solstices of June and December, respectively- 

 At Toronto the minimum occurs near the December solstice ; at the Cape near 

 the June solstice ; while at Toronto the maximum range happens two months 

 after the June solstice, it occurs two months after the December solstice at the 

 Cape of Good Hope. 



A similar resemblance of opposition may be traced betwixt the northern and 

 southern intertropical stations. Thus, at Simla, Bombay, Madras, and Trevan- 

 drum, the minima of range occur in October or November, and in February or 

 March ; while at Singapore and St Helena they occur in April or May, and in 

 September. That is to say, in the northern stations one or two months before 

 the December solstice, and near the vernal equinox ; while at Singapore and St 

 Helena they happen one or two months before the June solstice, and at the 

 autumnal equinox. In like manner, the maxima occur in the stations from 

 Bombay to Trevandrum in August or September, with the secondary maximum 

 in January or December ; while at Singapore and St Helena the secondary 

 maximum occurs in August, and the principal maximum in February.* 



General Remarks. 

 When we consider the curves for each station separately, we perceive that 

 they pass gradually from one form to another from month to month — the changes 

 being generally a gradual shift of the critical hours, and a variation of the relative 

 values of maxima and minima. The chief exception to this remark is to be found 

 in the change at Trevandrum in the month of March. In the Indian group, and 

 at stations even farther north, which I have not considered in this paper, the 



* At all the stations there is a kind of double in the epoch of principal maximum, seen more 

 or less markedly in May or June, and in August, in the northern stations ; in October or November, 

 and in February, in the southern stations ; at Singapore the range for November is slightly greater 

 than that for February. I should also notice, as a deviation from the law of change of minimum 

 from one station to another, the occurrence of the minimum oscillation at Madras in December, in- 

 stead of November, as its position betwixt Bombay and Trevandrum would indicate. I feel inclined 

 to believe that this deviation would not have appeared had the ranges for Bombay and Madras been 

 derived from observations during the same years. 



