ON SELF-RECORDING MAGNETOGRAPIIS. 201 



superior definition and finish of the lines leaves hardly anything to be 

 desired. Mr. Beckley, the engineer attached to the observatory, very skilfully 

 devised the mechanical details in conformity with Mr. Welsh's plan, and 

 prepared a working drawing of the instruments*. 



Mr. Chambers (magnetical assistant at Kew Observatory) assisted in over- 

 coming certain photographical difficulties that arose. He lias since been in 

 charge of the instruments, and has performed his task in a very efficient 

 manner. 



This Report is divided into five sections. In the. first section a general 

 and preliminary description is given of the principles of construction of the 

 magnetographs. In the secoud, a detailed account is given of each of the 

 instruments. In the third section the photographic process is described. In 

 the fourth, the method of ascertaining the instrumental coefficients, and of 

 tabulating from the curves, &c, is detailed ; and in the fifth section certain 

 improvements are mentioned which have been made on a set of magneto- 

 graphs since constructed of the same kind as those described. 



Section I. Preliminary Description. 



The room in which the instruments are placed is one of the lower rooms 

 of the observatory, the roof of which is not much above the level of the 

 ground outside. It is well protected from damp by a vault which goes 

 round the observatory, and is subject to very small changes of temperature, 

 the mean daily range being within 1° Fahr., and the annual range about 

 20°, the thermometer varying from 50° Fahr. in winter to 70° Fahr. in 

 summer. In shape the room is an octagon, of about 22 feet in diameter, 

 with a height of about 17 feet. Daylight is only admitted through panes 

 of orange-coloured glass, which have the effect of excluding the actinic rays. 



Four pillars, A, B, C, D (see Plate 3. fig. 1), made of Portland stone, are 

 firmly fixed into the Hoor. The centres of the pillars B, C, D are in a line 

 perpendicular to the magnetic meridian, while the centres of pillars A and D 

 are in the line of that meridian. The pillars A, B, and C support the three 

 magnetographs, while the pillar D supports the recording cylinders and 

 clockwork. 



In Plate 3. fig. 1, we have a ground-plan of the instruments, and in fig. 2 

 an elevation of the same. 



Referring to the Declination Magnetograph (Plate 3. fig. 1), a denotes 

 the gas-flame which is the source of light; b is a bull's-eye lens, the object 

 of which is to condense the light on a narrow vertical slit at c. The bull's- 

 eye therefore enables the light to be nearly as effective as it would be if 

 placed immediately behind the slit c, although in reality it is at a convenient 

 distance from it. 



After having passed the slit c, the light is conveyed through a covered 

 tube until it reaches the plano-convex achromatic lens set vertically at d, 

 having passed through which, it next falls on two semicircular mirrors which 

 have their centre at e. The faces of these mirrors are exhibited in Plate 4. 

 fig. 3, from which it will be seen that the lower mirror is firmly fixed to a 

 marble slab, while the upper one, which is nearly, but not quite in contact 

 with the lower, is attached to a delicately suspended magnet, and conse- 

 quently moves with it. The light, after leaving the mirrors, is reflected in 

 the direction (/"through a piece of plane glass at/, and through a covered 

 tube until it reaches a cylinder h, the axis of which is horizontal, and which 

 is covered with sensitive paper. 



The focal length of the lens d is such, that the point /*, where the rays 

 * The drawings for the Plates attached to this Report were also made by Mr. Beckley. 



