MAGNETIC DECLINATION NEAR THE MAGNETIC EQUATOR. 685 



pore and St Helena ; it happens in October at Trevandrum ; in November at 

 Bombay, and in December again it has returned to Simla. The minimum move- 

 ment of the free needle thus passes from latitude 31° N. (or it may be somewhat 

 north of this latitude) in December and January, to the equator in March and 

 April, and thence to 43° S., in June and July, recurring near the equator in 

 October, and returning to Simla in December. 



Maximum Ranges. 



The maximum movement of the free needle occurs in January at Hobarton ; 

 in February and March, at St Helena (or between St Helena and the Cape) ; in 

 April, May, June, July, August, and September, at Bombay or Madras (or perhaps 

 betwixt these two stations) ; in October at St Helena ; and in November and 

 December at Hobarton. The maximum movement changes its locality with the 

 period of the year, like the minimum, but it does not proceed so far north ; in 

 April and June, perhaps, it approaches Simla. The years from which the 

 Simla results are derived give probably somewhat less than the mean values ; 

 those for Madras, on the other hand, give values somewhat too great. 



The maximum movement of the free needle never occurs near the equator, as 

 at Trevandrum and Singapore, but passes suddenly, during the minimum move- 

 ments at these stations, from Bombay to St Helena, or from St Helena to Bom- 

 bay. The greatest movement of the free needle at any station occurs at Bombay 

 in August and September, and is equal to 5'-42, which is equivalent to a mean 

 diurnal oscillation at Toronto of the horizontal needle of 20'-9 ; while the greatest 

 monthly mean diurnal oscillation of the horizontal needle at Toronto, deduced 

 from the observations for 1842 to 1848, including the disturbance years of 

 1847-48, is only two-thirds of this quantity. In whatever way we combine the 

 results to represent the mean movement in each hemisphere, it seems to me 

 probable that the mean movement of the free needle for the whole earth is great- 

 est in August. 



The smallest movement at any station occurs at Simla and Trevandrum ; at 

 the latter from a combination of different movements. The least and greatest 

 movements of the free needle occur nearly under the tropic of Cancer. 



When we take the mean of the monthly ranges for each station, we find 

 Toronto to have the smallest mean, and next the equatorial stations of Trevan 

 drum and Singapore.* 



* It should be remembered, that we are examining the ranges of the monthly mean diurnal 

 variations, and that these are diminished near the equator, by the change of law occurring in months 

 near the equinoxes. 



VOL. XXIV. PART III. 8 Z 



