THE MAGNETIC DECLINATION AT ST. HELENA. 
55 
trary, the diurnal variation at St. Helena partakes in, and possesses the characters of 
the phenomena of both hemispheres, but each predominates in its turn, prevailing 
separately and in opposite seasons. The passage from one order of the phenomena 
to the other takes place at or soon after the period of the equinoxes ; in March and 
April, September and October, the diurnal variation at the hours referred to partakes, 
more or less on different days, of the characteristics of both seasons : but the months 
of May, June, July and August, on the one hand, and those of November, December, 
January and February, on the other, arrange themselves in wholly distinct categories; 
the north end of the magnet reaching its eastern extreme in the one case, and its 
western extreme in the other, nearly at the same hours ; the extremes being more- 
over in both cases nearly equidistant from the mean position of the magnet in the 
respective months. 
In Plate III. figs. 1 and 2 exhibit the projections of the diurnal variation at 
St. Helena from 3 a.m. to 3 p.m., in each month of the year, the projections repre- 
senting the mean of five years of observations. In fig. 1, which contains all the 
months excepting March and April, September is seen to belong on the whole to 
May, June, July and August, although the influence of the opposite season is plainly 
visible at the hours of 18 and 19 (6 and 7 a.m.). October, on the other hand, must be 
classed with November, December, January and February, although an influence of 
opposite character is distinctly perceptible in the direction from 1 7 1 * to 18 h (5 to 6 
a.m.)*. In fig. 2, the projections corresponding to the months when the sun has 
northern declination are collected into one darker line, as are those of the other five 
months when the sun has southern declination into another, for the purpose of ex- 
hibiting by comparison with the separate projections for March and April (which are 
the fainter lines), the degree to which the latter are intermediate. March and April 
have each both an eastern and a western elongation in the early morning hours ; the 
eastern occurs one hour earlier than in May to September, and the western one hour 
later than in October to February : these peculiarities, both in regard to direction and 
to hours, are traceable without difficulty to the occasional alternation in those months 
of the influences of the opposite seasons. 
Fig. 3. Plate IV. exhibits in the unbroken line the mean diurnal variation of the 
whole year at Toronto, and in the broken line the mean variation of the whole year 
at Hobarton in Van Diemen Island, showing the contrast which the opposite hemi- 
spheres present in this respect. In like manner the broken and unbroken lines in 
fig. 4 present in contrast with each other the diurnal variation of the opposite seasons 
at St. Helena. By the comparison of figs. 3 and 4, the general resemblance is shown 
* The fact that October corresponds much more to November, December, January, and February than to 
May, June, July, and August is very decided; and is important to notice, because, although the sun passes to 
the south of the equator in September, he continues to the north of the parallel of St. Helena until the begin- 
ning of November. It is the sun’s position in reference to the earth’s equator, therefore, and not to the zenith 
of the place of observation, which marks the epoch of the change in the direction of the diurnal variation of 
the needle. 
