194 TEAK-BOOK OP FACTS. 



upon which this instrument is constructed, taken from the Journal 

 of the Photographic Society : — 



A pencil of light, brought to a focus by spherical or cylindrical lenses or reflec- 

 tors, is so governed that its point or focus has a motion identical with, or bearing 

 a known proportion to, the motion of part of the instrument which affords the 

 indications to be registered. Thus, if the instrument be a magnetic needle, the 

 axis of the lens or spectrum is made to coincide, or make a known or constant 

 angle, with the needle, and therefore to participate in its movements. The focus 

 of the pencil refracted or reflected receives a corresponding motion. If it be a 

 column of mercury, as in the case of a barometer or thermoi r, the direction 

 of the pencil of light is varied, either by means of a float, which rises and falls 

 with the mercurial column, or by transmitting the light through the tube, so 

 as to produce the shadow of the column, in which case the movement of the 

 shadow will be registered. The focus of the luminous pencil is made to 

 fall upon a sheet of photographic paper; and if both it and the paper were 

 stationary, a spot would be produced upon the paper at the place where 

 the focus falls upon it. If, owing to the variation of the instrument whose 

 indications are to be recorded, the focus of the luminous pencil moves, a line 

 will be traced on the photographic paper, the length of which will boar a Known 

 relation to the variation of the instrument. Thus, if it be a magnetic needle, 

 a variation of one degree east or west in its direction may impart a motion 

 of an inch right or left: to the focus of the luminous pencil, and a line of cor- 

 responding length would be traced upon the photographic paper. Hut by this 

 means nothing would be recorded, except the extreme variation of the 

 needle in a given time. An observer woidd still be nece isaxy : and nothing would 

 bo accomplished more than is already attained by the self-registering thermo- 

 meters, which show the r.iarimum.ancl minimum temperatures indicated during 

 a given interval. 'I - is, however, rendered perfect by rolling the 



photographic paper on a cylinder, which is moved by clockwork, so that a 

 under the focus of the luminous pencil in a 

 given time. When the focus of the pencil is stationary, a straight line is traced 

 on the paper in a direction at right angles to the motion of the paper, and there- 

 fore parallel to the axis of the cylinder; but when the focus moves, as usually 

 happens, to the right and left alternately, an undulating cu . upon the 



paper, the distances of the points of » bioh from a known base line (also traced 

 upon the paper) show not only the particular minute and second at whii 

 change took place, but the actual state of the instrument at that moment. 



The Edito*r of the same journal states that a beautiful apparatus, 

 constructed for Kew Gardens, is the most reliable barometer ever 

 yet invented. By means of a wire communicating with the atmo- 

 sphere on the one hand, and a very delicate electrometer on the 

 other hand, a .silvered bead, placed at the end of a thread of glass as 

 a silkworm fibre (the glass thread being the most elastic of all 

 substances), i-; carried to the right or to the left hand in the ratio of 

 the amount of vitreous <>r resinous electricity. The bead rel! 

 a spot of light on to sensitive paper moved in the way before de- 

 i permanent record of every change in the electri- 

 cal condition of the atmosphere, day and night. As an elucidation 



of its use, the writer mentions a hot as gratifying aa it will bo 



a toundingl » many of our readers : The day of the ration 



of Her birthday was one of thi of very numerous 



wet days with which it has lately pleated Providence to visit us. 



i Thomson, of Glasgow, was lecturing 



apon this electro-photographio apparatus at the Royal Institution. 



During the greater part of the lecture the rain wasominou ly patter* 



■ >;' the theatre. At the conclusion <>f his l< otore he 



led to take an observation by the means of a wire passing 



