164 Mr Christie on the Variation of the Magnetic Needle . 
gulating index, the last thing he has got to do is to open the 
injection-cocks, and immediately the engines start in the direc- 
tion wanted. 
Thus we have, at your request, endeavoured to sketch and 
explain to you such parts of the ferry twin-boat, George the 
Fourth, of this place, as you more particularly wanted informa- 
tion respecting. We hope that it is done in such a way as you 
will understand it ; but if any further explanation is wanted, be 
so kind as write us. — We are, Sir, your very obedient servants,. 
Jas. and Chas. Carmichael. 
Art. XVII. — On the Diurnal Variation of the Needle . Bj 
Samuel Hunter Christie, Esq. M.-A.* 
The diurnal variation, both in the direction of the needle 
and in the magnetic intensity, appears to have a reference to the 
position of the sun with regard to the magnetic meridian ; it is 
therefore probable, that the sun is the principal cause of both 
these phenomena. The circumstance of the situation of the 
magnetic pole in what appears to be, independent of elevation,, 
the coldest region of the globe, supported as it is- by the fact of 
a diminution of temperature causing an increase of magnetic 
intensity, would lead us to infer, that the effect produced by the 
sun is principally to be attributed to the heat developed by it 
but should any periodical effects, corresponding to the time of 
the sun’s rotation about its axis, be observable in the diurnal 
variation, we must suppose that the sun, like the earth, is en- 
dued with magnetism, and look for a cause of this magnetism^ 
common to all the planets. Being engaged more than two years 
ago in making some experiments on the effects produced on the 
needle by unpolarized iron, I discovered that a peculiar pola- 
rity was imparted to the iron by simply making it revolve about 
an axis ; and this naturally suggested the question to me, whe- 
ther the magnetism of the earth, and, consequently, that of the 
other planets and the sun, might not be owing to their rotation ? 
* From his memoir u On the effects of the intensity of magnetic forces, and 
orr the diurnal variation of the terrestrial magnetic intensity.” 
