THE SMITHSONIAN INSTITUTION. 389 



The cylinder is 12 inches long, and as the space described on its 

 surface, by a movement of the magnet of minute in arc, is equal to 

 one-fifteenth of an inch — the distance to the mirror being 9| feet — 

 we have room for recording variations to the extent of 180' or three 

 degrees, which will be approached only in extraordinary disturbances. 

 The diameter of the cylinder is 85 inches, and it revolves by the 

 action of a time-piece once in twenty-four hours, giving a time-scale 

 of a little over one inch for each hour. The cylinder is made of light 

 staves of white pine, truly turned on its axis, and coated with shellac 

 varnish, which protects the sensitized paper from the reaction of 

 organic acids in the wood. In order to secure still further this pro- 

 tection, the cylinder is covered by a sheet of stout drawing paper, 

 which is replaced by a new one from time to time. Mr. Brooke uses 

 glass cylinders, selected from Fiench glass shades for covering vases, 

 &c; and this material certainly has the advantage of not in any way 

 reacting on the sensitive preparations, and of being most readily kept 

 clean. But it is so difficult to obtain them of true and uniform figure 

 that the means before mentioned have been resorted to, and with 

 good success. Of a dozen glass cylinders sent with the instrument, 

 but two arrived unbroken, and these are used for the vertical force 

 register, while the wooden cylinders are used for the declination and 

 horizontal force. Each cylinder is driven by a clock-train, regulated 

 by a seconds pendulum with wooden rod, and revolves once in twenty- 

 four hours. It is covered with a blackened copper case, having a 

 narrow slit through which the pencil of light strikes the paper, as 

 seen in figure 3. 



PHOTOGRAPHIC PROCESS. 



The record paper is prepared by immersing it first in a solution 

 of iodide of potassium — half an ounce of iod. potass, to ten ounces of 

 water. After drying it thoroughly in the air, one side of the sheet is 

 carefully floated on a solution of nitrate of silver — 320 grains of 1 used 

 gray lunar caustic, (or nitrate of silver,) perfectly free from acid, in 

 10 ounces of water — for \\ minute, and after being allowed to drain 

 a little while by being held up by one corner, it is washed in several 

 changes of water, for which cistern water will do well enough, and 

 hung up to dry in the dark. A number of sheets may be prepared 

 at one time in this manner, and preserved for use in a dark place for 

 ten or twelve days. 



When required for use one of these sheets is wrapped about the 

 cylinder, which for that purpose is taken out of its bearings and 

 placed upon similar supports attached to a table in the laboratory. 

 It has been customary to fasten the ends by a paste made of gum- 

 arabic dissolved in acetic acid; but two elastic bands clasped around 

 the sheet, one at each end, are found quite sufficient, and much more 

 convenient. The cylinder being replaced in its bearings, and con- 

 nected with the clock movement, a mark is drawn across the edge or 

 joint of the sheet with a hard pencil, which, when the sheet is un- 

 rolled, furnishes points for drawing a line of abscissas, on which the 



