MAGNETIC DECLINATION NEAR THE MAGNETIC EQUATOR. 687 



curves present an apparent anomaly in the movement betwixt 7 a.m. and noon 

 for the four months November to February, which is a combination of the move- 

 ments in high north and south latitudes at the same period. Yet we find that 

 this form continues in a general way throughout the year, the maximum shifting 

 from 9 h 30 m a.m. to 7 h 30 ra a.m. 



It appears from the first part of this paper, that the law of movement varies 

 with great rapidity from day to day near the magnetic equator about the time of 

 the equinoxes, and especially in the month of March, when the mean curve at 

 Trevandrum approaches the straight line. From the second part it appears that 

 the law of movement varies rapidly with the latitude at the same time of the 

 year near the equator. There is a zone of disturbance near the equator when the 

 sun passes from one hemisphere to another. It will require investigations at 

 other stations to determine how far this zone extends. If we may judge from 

 the epochs of minimum movement, it would appear that the zone is north of the 

 equator when the sun is in the southern hemisphere, that it reaches the equator 

 with the sun when the disturbance has the greatest intensity, and that it passes 

 south of the equator as the sun moves northwards. 



The number of stations should be increased betwixt the latitudes of St Helena 

 and Singapore, to show clearly the mode in which the law changes near the equi- 

 noxes from one station to another, though there can be little doubt that a station 

 midway betwixt the two would show movements represented nearly by the mean 

 of the curves for St Helena and Singapore. I have myself made observations of 

 magnetic declination simultaneously at Trevandrum, at a station ninety miles 

 north on the magnetic equator, and at Cape Comorin, forty miles south of 

 Trevandrum, during the equinoxes. I shall hereafter show the incremental 

 curves due to these changes of latitudes.* 



I have already noticed a fact pointed out by me in the Makerstoun Observa- 

 tions for 1844,-f- that the difference of ordinates of the curve representing the 

 summer movement at Makerstoun, and of that representing the winter movement 

 at the same place, gave a curve representing the typical movement of the southern 

 hemisphere; so that the relation of the summer curve at Makerstoun to the 

 winter curve at the same station, was the same in kind as that betwixt the two 

 opposite curves seen near the equator. In other words, the summer curve in 

 high north latitudes was only diminished in range by the change which sufficed 

 to invert the curve for the same epoch at the equator. J 



* Observations were also made at a height of 6000 feet above Trevandrum, and twenty miles 

 W.N.W. of it. 



f Trans. Roy. Soc. Edin. vol. xviii. p. 354. 



j It is not meant that the change was the same in amount in high latitudes and near the 

 equator, though, when we consider the variations of the horizontal needle at Makerstoun and Tre- 

 vandrum, the changes are nearly equal. 



If we subtract the curve with maximum movement from that of minimum movement of the 

 free needle at each station in the higher latitudes where the curves follow nearly the same law, or 



