76 



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

 light some points 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 phenomena 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 phenomena, 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 I'-g- east of it ; in October it has passed the mean line, being 

 about 1'f west of it; November, December, January and February 

 are congregated near the western extremity of the annual range, 

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

 ing the mean line, and in April it has passed to the east of the mean 

 line. "We have here, then," the author proceeds, "in the success- 

 ive changes of the declination in the course of the year, the general 

 fact of the existence of an annual variation, of which, at the solar hour 

 of 7 A.M., selected as an example, or when the sun is five hours east 



