146 Royal Society. 



change in the southern hemisphere from that in the northern, which 

 takes place in the opposite direction ; whilst from the form of the 

 isogenic lines in that quarter, we may further anticipate that, at St. 

 Helena, the secular change of the Declination will continue to take 

 place in the same direction as at present, until the line drawn through 

 the conical summits of the isogonic curves shall in its western pro- 

 gress pass the geographical meridian of that station." 



The author then proceeds to the Variations which are found to 

 take place in periods corresponding to a solar year and a solar day ; 

 a correspondence which, he remarks, " enables us to recognize a phy- 

 sical connexion, although we are still uncertain as to the mode of 

 operation between cause and effect. A correct knowledge of the 

 phsenomena themselves is the surest guide to a correct judgement 

 amongst the many theories which have been propounded in anticipa- 

 tion of that knowledge ; and I have therefore taken this opportunity 

 of bringing before the Society a careful analysis of the primary an- 

 nual and diurnal variations at St. Helena attributable to solar influ- 

 ence, in the belief that they will be found to place in a very distinct 

 light some {)oints which are important to be kept in view in framing 

 or in judging of such theories." For this purpose diagrams were 

 exhibited, representing on a large scale the mean diurnal variation 

 of the Declination at St. Helena in the different months of the year, 

 and the annual variation at each of the twenty-four hours, both de- 

 rived from the mean of five years of hourly observation ; the secular 

 change having been previously eliminated, these diagrams were re- 

 garded by the author as exhibiting what might be considered as 

 typical views of the annual and diurnal variations, correct in their 

 relations to the mean Declination in the year, or to the arithmetical 

 mean of all the hourly observations in the year, taken as zero. As 

 on the first aspect the diurnal phsenomena in the several months are 

 seen to separate themselves into two groups, having the equinoxes 

 as at least approximate epochs of separation, the months in which 

 the sun is north of the equator were coloured red, and those in which 

 he is south of the equator were coloured blue. 



Having in these diagrams the conjoint representation of two 

 distinct classes of phaenomena, a diurnal variation in each of the 

 months, and an annual variation at each of the hours, the author 

 proceeded to treat of each of these variations separately, commencing 

 with the annual, which he illustrated by taking the hour of 7 a.m. 

 as an example, and (referring to the diagram) showing the order 

 and succession of the several months in the annual cycle at that 

 hour, which are as follows : — in April the mean declination is about 

 half a minute east of the mean declination in the year ; in May about 

 2' east; in June about 2'^ east; in July and August, when the 

 sequence is slightly irregular, respectively 2'*1 and 2'' 6 east ; in Sep- 

 tember the declination is again approaching the mean line, being 

 less than 1'^ east of it ; in October it has passed the mean line, being 

 about l'^ west of it; November, December, January and FebruEiry 

 are congregated near the western extremity of the annual range, 

 whilst in March we perceive that the declination is again approach- 



