ON SELF-RECORDING MAGNETOGRAPHS. 227 



paper. These sheets are therefore only changed every second day. This 

 change is made a little after 10 a.m., and the time occupied in making it is 

 about ten minutes, while that occupied in pushing forward the dots is only 

 about three minutes. There is thus every day a loss of ten and of three 

 minutes alternately, so that the curves never record precisely the whole of 

 the twenty-four hours, but generally something less by a few minutes. The 

 precise moment (Kew mean time) of stopping the pendulum and of setting 

 it going again is noted, so that the length of time for which any curve is 

 a record is known and is attached to the curve in writing. (See curves 

 appended to this Report, Plate 5.) 



The instrument for tabulating from the curves is represented in Plate 3. 

 fig. 3 a : ab is a time-scale commencing and ending with 22 h . This scale is 

 moveable round a as a centre, and the centre a is also moveable in a hori- 

 zontal direction. Part of the instrument, dfg, is moveable in a vertical 

 direction by means of h, the head of a pinion which works into the rack i; 

 d serves as a vernier for the scale e. The piece c d efg is moveable in a 

 horizontal direction by means of a slide which fits into the slot k I; /and g 

 are two tubes through which the eye looks at lines on a piece of glass (ex- 

 hibited separately at full size in fig. 3 «). These are two sets of double lines 

 which are etched on glass,the sets beingexactly two inches apart. The distance 

 between the tubes/ and g is also two inches, so that when the upper pair of 

 lines is placed under /, the lower pair is under g. The glass is firmly 

 attached in this position to the moveable piece dfg, so that the double lines 

 remain exactly under the tubes in whatever manner dfg is moved. The 

 breadth between the two lines (which together constitute a double line) on 

 the piece of glass is a little greater than the breadth of the curve or zero- 

 line on the photographic paper. 



In order to measure the distance between the curve and zero-line, the 

 photographic paper is set between two pieces of plate-glass, and so adjusted, 

 that when the tube g is set over the zero-line, it may continue to be approxi- 

 mately over it in any part of its horizontal range. 



Suppose now that c d efg is at the extreme left, the vertical line of the 

 piece of glass lying along the commencement of the curve and that of the 

 zero-line. Set the time-scale ab so that the edge of the index e may touch 

 that hour on the time-scale which corresponds to the commencement of the 

 curve. Adjust the vertical height of b, the extremity of the time-scale, so 

 that when c d ef g is carried to the other or right-hand extremity of the 

 curve, the index c may touch that division of the time-scale which corresponds 

 to the termination of the curve. Were the same length of base-line always 

 to denote the same space of time, there would be no need of altering the 

 inclination of a J; but the rate of the clock may vary a little, or the paper 

 may fit more or less loosely to the cylinder, so that an inch of the base-line 

 will not always denote precisely the same space of time. Having thus 

 adjusted the time-scale, in order to find the distance between the base-line 

 and the curve for any hour, set the index c to the required time, move the 

 pinion head h until the upper pair of etched lines at/ are over the curve-line, 

 and read off the height on the scale e by means of the vernier d. Next move 

 the pinion head h until the lower pair of etched lines at g are over the base- 

 line, and read off by means of the vernier as before. The difference between 

 the readings for the curve and the base-line plus two inches, gives the distance 

 between these lines. 



In case any shifting should take place, it is best to read the curve and its 

 corresponding base-line consecutively, instead of reading first a number o 

 points of the curve together, and then the corresponding points of the base- 



Q2 



