METEOROLOGY. 249 
[t still remains to mention an occasional electric phenomenon known at 
the present day as St. Hlmo’s Fire (and to the ancients as Helena, or Castor 
and Pollux). During the disturbed condition of atmospheric electricity in 
storms, and at other times, flames are observed on elevated objects, such as 
metal points of towers, mast heads, &c., which are sometimes heard to 
crackle along these objects, but without doing any injury. This is nothing 
more than electrical light streaming forth during a great accumulation of 
free fluid, and is especially observed during violent storms, as also in snow 
and hail squalls. The ancients, when they saw two of these lights (Castor 
and Pollux) on the tops of the masts, considered them as indicative of fair 
weather, while a single one (Helena) was supposed to portend a storm. 
This phenomenon is presented more frequently in winter than in summer, 
and is sometimes exhibited in the former by a luminosity of the snow flakes, 
in the latter by the same peculiarity in the descending rain or hail. When 
the amount of free electricity is very great, it is seen on low objects, stalks 
of grass for instance, as observed by Burchell in South Africa; also on lance 
heads, canes, finger tips, ears and manes of horses, &c. The balls of fire 
sometimes observed, as distinguished from the flames or stars, may be the 
exhibition of negative electricity, the latter being positive. 
9. Of Terrestrial Magnetism. 
The influence exerted by terrestrial magnetism, at any locality on the 
earth, is determined by measuring the magnetic declination, inclination, and 
intensity, as its three exponents. For the general consideration of this 
subject we would refer our readers to what is said on page 141. To 
determine the declination and its variations, the method of Gauss, and the 
accompanying apparatus, the magnetometer, are almost universally used. 
In this instrument, instead of small magnetic needles, magnetic bars of from 
five to twenty-five pounds’ weight are employed. A bar of this character. 
eighteen to thirty-six inches long, three to six lines thick, and fifteen to 
twenty-four lines broad, is placed on a nut of brass, which is suspended from 
the ceiling of a room by a fine wire, or thread of untwisted silk, about seven 
feet in length. A plane mirror is fastened to one end of the bar, whose 
plane stands perpendicular to the magnetic axis of the bar: opposite to the 
mirror, but at a distance of about sixteen feet, a telescope is attached, whose 
optical axis inclined slightly downwards is directed immediately towards the 
centre of the mirror. The angle made by the optical axis of the telescope 
with the plane of the astronomical meridian must be determined with the 
greatest possible accuracy. A horizontal scale about four feet long, 
graduated to millimetres, is attached to the stand of the telescope, perpendi- 
cularly to the direction of the magnetic meridian, and at such a height that 
the image of a part of it is seen by reflection in the mirror. <A thread 
stretched by a weight, and in contact with the scale, hangs before the 
middle of the objective indicating the middle or zero point of the scale, that 
is, the point which lies in one and the same plane with the optical axis of 
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