73 



2. " On the Annual Variation of the Magnetic Declination, at 

 different periods of the Day." By Lieut.-Col. Sabine, R.A., V.P. 

 and Treas. R.S. &c. Received April SO, 1851. 



In this communication the author has arranged and presented 

 together the Annual variations which the magnetic declination 

 undergoes at every hour of the day at the four Colonial Observatories 

 established by the British government, at Toronto, Hobarton, the 

 Cape of Good Hope and St. Helena. This has been done by 

 means of a graphical representation, in which the annual variations 

 at every hour are shown by vertical lines varying in length accord- 

 ing to the amount of the range of the annual variation at each 

 hour ; each line having also small cross lines marking the mean 

 positions of the several months in the annual range. The mean de- 

 clination in the year at the respective hours is marked by a hori- 

 zontal line which crosses all the verticals at each station. The 

 hours are those of mean solar time at each station, the day com- 

 mencing at noon. The annual variations represented in the plate 

 were obtained at Toronto from three years of observation, viz, 1845, 

 46, 47 ; at Hobarton from five years, viz. July 2nd, 1843 to July 1, 

 1848; at the Cape of Good Hope from five years, viz. July 2nd, 

 3 841 to July 1st, 1846 ; and at St. Helena from three years, viz. 

 July 2nd, 1844 to July 1st, 1847. 



The author observes that it is perceived at the first glance at the 

 plate, that the range of variation at all the four stations is consider- 

 ably greater during the hours of the day than during those of the 

 night ; and that there is a great similarity, though not a perfect 

 identity, at all the stations in the relative amount of the range at 

 different hours. Further, that the amount does not progressively 

 enlarge to a maximum at or about noon, when the sun's altitude is 

 greatest ; or at the early hours of the afternoon, when the tempera- 

 ture is greatest; but that at all the stations the increase of the range 

 is most rapid in the first or second hour after sunrise ; and that its 

 extent at the hours from 7 to 9 a.m. is not exceeded at any subse- 

 quent hour at Hobarton, the Cape and St. Helena, whilst at Toronto 

 the great enlargement takes place even earlier, the hours of 6, 

 7 and 8 a.m. being exceeded by none, though they are equalled by a 

 second increase at noon and the two following hours. This second 

 <:'nlargement is perceptible at the same hours at Hobarton and St. 

 Helena. 



With reference to the relative positions of the several months in 

 each of the vertical lines, or at the different hours, it is observed that 

 certain months, which are found congregated at the one extremity 

 of the range during the early hours of the morning, undergo a 

 transfer towards the opposite extremity at a subsequent period of the 

 day ; thus the months June, July, August usually occupy one 

 extremity of the range, and November, December, January the 

 other extremity, in the morning hours, and until from 8^ to 10^ 

 A.M., when each of the two groups is respectively transferred 

 towards the opposite extremity to that which it previously oc- 

 cupied. The period at which this transfer takes place is somewhat 



