248 Magnetic Influence of Solar Light, 



one-half of the needle, invariably from right to left, a spe- 

 cified number of times. The exposed end, for the sake 

 of clearness, will always be styled the marked end, it having 

 been distinguished from the other by a small black dot, this 

 other end, being the unmarked one. After exposure, the 

 needle was replaced in its stirrup, and allowed to take up a 

 fixed and settled position, so that there might be no remains 

 of torsion in the silk filaments to interfere with the freedom 

 of its vibrations. It was then carefully adjusted, so as to 

 coincide with the zero line of the graduated circle, its centre 

 being placed exactly over the centre of the circle ; a slen- 

 der rod of straw introduced within the glass shade through 

 the aperture at its top, and by means of this rod a slight 

 impetus was communicated to, the needle, by which its 

 oscillations were established. These were allowed to dimi- 

 nish until the arc had a semi-amplitude of 54°, from which, 

 as from a fixed point, the counting and registering com- 

 menced in every case. The time was registered every 60 

 seconds by the aid of an excellent watch, although from 

 this not having a stop, it was a little difficult to read off 

 with the exactness required. The results in the following 

 table are given as they were obtained at the time of experi- 

 ment, with the exception, that whereas I then endeavoured 

 to register the amplitudes of the first and last oscillations, 

 I found it afterwards so difficult to attend to both these 

 and the times, that I struck out the former and attended 

 wholly to the latter. The temperature, as being an element 

 of some importance in experiments such as the present, is 

 given in degrees of Fahrenheit, the needle was frequently 

 very warm after exposure, but invariably became perfect- 

 ly cold again long before the experiments with it were 

 closed. 



I now proceed to give numerical details, and adopt the 

 tabular form, as being in every respect the most convenient 

 and concise. 



