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



case there is a near approximation to the mean diurnal variations for each month 

 at each station, so near that the addition of any number of years' observations 

 would not alter in any way the conclusions of this paper. 



Comparison of the Mean Diurnal Variations at different Stations. 



The monthly mean diurnal variations for each station having been reduced to 

 those of the needle freely suspended in the direction of the magnetic inclination, 

 the resulting values were projected in curves, Plate XLIV., which we shall proceed 

 to consider month by month. 



January. — The minimum of easterly declination (north end of needle farthest 

 west) occurs from Bombay to Hobarton at beween 7 a.m. and 9 a.m. At Simla, 

 there is a minimum at 7 a.m., but rather less marked. This minimum evidently 

 diminishes gradually in importance as we proceed north, being shown at Maker- 

 stoun in a very faint degree only. This minimum is followed by a maximum occur- 

 ring betwixt 9 h and 9 h 30 m a.m., which also becomes less marked as we proceed 

 towards Makerstoun, where it appears as an inflexion on the descending branch 

 of the curve ; it disappears in like manner south of Trevandrum, where it is seen 

 as a slight maximum on the ascending branch. The minimum near 8 a.m. can, 

 therefore, be traced from Hobarton to Makerstoun, while the maximum near 

 9 h 30 m a.m. appears only north of the equator. From Makerstoun to Trevandrum 

 the maximum is followed by a minimum (the principal minimum from Simla 

 northwards) betwixt l h 30 m p.m. and noon. This minimum, which is so well 

 marked from Madras northward, almost disappears in the next 4°| of latitude, 

 being seen faintly at Trevandrum. At Singapore it is no longer visible. 



At Trevandrum and the stations south of it, the principal maximum of easterly 

 declination is attained near 2 p.m., excepting at St Helena, where it occurs near 

 noon. As marked, a displacement occurring betwixt Singapore and St Helena 

 as betwixt Singapore and Madras. In the latter case, the mode in which the 

 transition happens is shown by the curve for Trevandrum ; as there is no station 

 betwixt Singapore and St Helena, the leap appears more abrupt. 



A secondary maximum and following minimum occur in the group Simla to 

 Trevandrum at 4 p.m. and 6 p.m., which become more marked in succeeding 

 months. St Helena and the Cape have a minimum at 5 and 6 p.m., followed by 

 a maximum near 9 p.m. At Makerstoun, the station farthest north, the maximum 

 occurs near 10 p.m. 



February. — The remarks made upon the curves for the preceding month apply 

 generally to those for this month. The curve for St Helena differs less from 

 those for Singapore and the Cape than before the maximum occurring betwixt 

 1 and 2 p.m. 



March and April. — In these months, when the sun is near the equator, the 



