162 Mr. C, Chambers on Magnetic Declination at Bombay. [Dec. 10, 



16th. The variation from year to year in the range of the mean diurnal 

 variation of declination 



1 7th. The secular change and semiannual inequality of absolute decli- 

 nation. 



The diurnal variations of disturbance, both easterly and westerly, are 

 found to be of definite and systematic character, and to be comparable with 

 the same variations for other places ; the annual variation is not very 

 regular, but the progression in the amounts of disturbance in different 

 years accords well (with exception as to the incomplete year 1861) with 

 the known character of the decennial variation. The mean diurnal varia- 

 tion of declination, as well as its semiannual inequality, is of the general 

 character due to the latitude in which Bombay lies ; the progression from 

 month to month in the annual variation of the diurnal variation is also di- 

 stinctly marked in all months except July. A semiannual inequality is 

 shown to exist in the diurnal variation of declination whose times of oppo- 

 sition are the equinoxes. It is found that this inequality not only exists, 

 but has the same general character at five widely separated stations in the 

 northern magnetic hemisphere, and also, with some modifications as to cha- 

 racter, at two stations having south magnetic latitude. Its special charac- 

 teristics are : — 



1st. That, as in the typical mean diurnal variation of declination, there 

 is scarcely any change during the night hours, and that the main varia- 

 tion occurs during half the day, in this case between 18 hours and 6 hours, 

 local astronomical time. 



2nd. That the range of variation differs from about half a minute to 

 nearly a minute of arc. 



3rd. That the hour of noon is that about which the deviations due to 

 this variation pass through zero, and on each side of which the inflexions 

 of the representative curve are inversely, but, in respect to north latitude 

 stations, symmetrically disposed. 



4th. The turning-points are 21 hours and 3 hours, the former being a 

 maximum, and the latter a minimum for north latitude stations from Ja- 

 nuary to June, and for south latitude stations from July to December ; 

 and vice versa, for north latitude stations from July to December, and 

 for south latitude stations from January to June. The solar-diurnal ine- 

 quality of declination, in the calculation of which all disturbances are in- 

 cluded, differs at no hour of the day by more than 0''"061 from the mean 

 diurnal variation, which is calculated after the rejection of all observations 

 disturbed to the extent of more than l'*4. 



The range of the diurnal variation of declination in different years is 

 shown to be subject to a periodical variation, whose times of maxima and 

 minima approach nearly to those of the maxima and minima of the decen- 

 nial period in the amount of yearly disturbance. 



The secular change of absolute declination is found for the years 1859 to 

 1865 to be an annual increase of easterly declination of 3'*017 ; the semi- 



