of Terrestrial Magnetism, 391 



only one which can faithfully and fully illuminate the labyrinth 

 of magnetic observation. I will briefly notice in what the appa- 

 ratus, which has been working since 1847, consists, and will 

 describe what was registered in the volume of the Meteorological 

 Observations of Greenwich for 1847, and which I saw in 1849 

 through the kindness of Messrs. Airy and Glaisher. 



The magnetic observatory is erected in a garden adjacent to 

 the astronomical observatory; and the instruments, of which 

 the march is photographically registered, are the declinometer, 

 bifilar and balance magnetometers, and, for meteorology, the 

 barometer, dry- and wet-bulb thermometers, &c. To speak 

 here only of the magnetic instruments, the principle of regis- 

 tration is the following. Each magnetic bar has attached to it 

 a small mirror, which receives through an aperture the rays 

 of a gas-light passed through vapour of naphtha^ to render its 

 photographic action more efficacious. The mirror reflects these 

 rays on a lens, which concentrates them in a bright point on a 

 prepared photographic paper. This paper is rolled round a 

 cylinder, which is moved by clock-work, and makes an entire 

 turn in twelve hours, having its axis of rotation parallel to the 

 line which the magnetic needle, reflected from the mirror^ tends 

 to trace as it vibrates. At the slightest oscillation of the needle, 

 the reflected ray changes its place on the paper, and makes an 

 impression on it; and this movement of the reflected image 

 being compounded with the movement of rotation, there is pro- 

 duced on the paper a curve, of which the ordinates represent the 

 amounts of the variations, and the abscissae their times. If the 

 needles are much disturbed, the pap^r is changed every twelve 

 hours; if not, two curves are obtained on the same sheet. In 

 order to have fixed points, and a base from which the times and 

 ordinates may be counted, there is a second light, which sending 

 its rays directly on the paper through a fixed hole, traces on it a 

 straight line, to serve as the axis of the abscissae ; slight inter- 

 ruptions are made in the curve at intervals, by intercepting the 

 light, so as to give points of departure for counting the times of 

 the movements without error, and independently of the possible 

 irregularities of the clock-movement. 



The sheets on which these curves are traced are fixed by known 

 photographical processes, and carefully preserved ; and there are 

 afterwards taken from them, by means of proper scales of reduc- 

 tion, the numerical data. Such a collection of observations 

 cannot be otherwise than highly profitable. Science however 

 expects its rigorous discussion with impatience. The apparent 

 irregularities, which are numerous, especially in the more northern 

 observatories, will disappear by the multiplication of curves ; and 

 although their first aspect may be discouraging, we shall see 



