_ 
Rg m I | m 
6 ie ) Bi ia ci 
NY Pee So ae 
ee 
chet (iy ™ ¢ ™m 
4. Se —— — 
and Directive Force of the Magnetic Needle. 195 
Diurnal Variation of the Declination, Toronto, 
12k 13h 14k 15k 16h 17h 18h 19h Qh Qh Bn Q3r Or 1h Qh Bk 4h Gh Gh Th Sh Qh 10k 11h 12h 
ree. é 
113k 14h 15k 16h 17h 18h 194 QOh VIA Qh 23i Oh Ir 2h Bh 4h 5h Gh 7h Sh Qh 1Oh 11h 12h 
Fig. 3. April to August, inclusive——Fig. 4, October to February, inclusive, 
01 answers to noon. Scale (i725 towl’ of are. Ascending curve is increasing declin- 
ation (West), descending dl ing declination: mm, line of Mean Declination. 
lie to the east of the prime vertical and to the north of the equa- 
tor in summer, and to the west of the prime vertical and south of 
the equator in winter. On the other hand about the hour of noon, 
en the points in question are in the vicinity of the meridian, the 
West declination ought to be greater in summer than in winter. We 
shall see that the ecliptic currents conspire with the radial in pro- 
ducing a relative depression in the morning. In like manner each 
System of currents tends to make the turning point, from easterly 
to westerly motion, occur earlier in summer than in the winter. 
To consider now the action of the radial currents alone; we have 
to observe that on all the parallels of latitude to the north of the 
equator the sun has a greater altitude at a given hour in the fore- 
hoon, in the summer than in the winter: hence the points in those 
Parallels, which lie to the west ofthe prime vertical, have a higher 
electric intensity in the summer. Thus, at Toronto, the turning 
Point in December is 9 a.m., in June 7 a. m.; now 9 a.m. in De- 
cember is 14 hours after sunrise, and in June 44 hours. The sun’s 
Htitude also increases more rapidly in June than in December. 
ere is another annual variation, which has recently been 
brought to light by Colonel Sabine. It consists in this: If a 
Comparison is made of the values of the declination at the hour 
7 A.M. among themselves, it appears that “at the north- 
€tn solstice the north end of the magnet is at the eastern extreme 
of a periodical movement, which apart from, and independently 
of; all other movements whatsoever, has its opposite or west- 
er eXtreme at the period of the southern solstice, and returns 
iNto itself at the next return of the northern solstice. It is there- 
- ges Strictly an Annual Variation, or a variation whose period is 
Year, Its amount is, at Toronto, about five minutes of declin- 
