24 Mr Watt on a New Magnetical Instrument^ 



but rather increased, by augmenting the number of the needles. 

 I have tried it with about three hundred needles, and obtained 

 the same results. 



It appears to be, to a certain degree, affected by the light 

 of the moon, when full, and also seemed at times, when first 

 formed, to be strangely influenced by the different states of 

 the atmosphere during the night, when the moon was not in 

 our hemisphere. Once or twice, when placed in the open air, 

 in a clear atmosphere, it continued for a considerable time to re- 

 volve upon its pivot, stopping occasionally, and then commen- 

 cing its circular movement. What occasioned this motion I 

 could not discover, except it was some change in the electrical 

 state of the atmosphere. 



This instrument, besides exhibiting clearly the power of solar 

 influence on magnetic needles, perhaps might be rendered a good 

 photometer. It might be used in climates where the sun is seldom 

 shrouded by clouds as a moving dial. Its dip, which is visible 

 when formed of one star of needles, may be useful at sea ; and I 

 conceive it is possible, that it may be ultimately made so sensi- 

 tive as to be attracted or repelled by the sun's influence through 

 its partial obscuration by mist or clouds, and then its utility at sea 

 would be of great service. The instrument is at present affect- 

 ed by the solar rays through thin clouds ; and if a piece of cork, 

 of six inches diameter, is cut into a circular shape, and its two 

 flat surfaces made a little convex ; and if about two hundred 

 magnetic needles are fixed into one of the surfaces of this lens- 

 shaped piece of cork, radiating from the centre like the petals 

 of a double anemony or daisy, and all the south poles of the 

 needles, placed so as to point to the circumference of the circle ; 

 and if this star of needles is suspended under a glass cover, they 

 are affected so far, by the light afforded by a window, that, whether 

 the sun shines clearly or not, the side of the cork on which the 

 needles are placed will not rest opposite to the light, but the cir- 

 cle will either turn its edge or the side without the needles to- 

 wards the window, before it remains stationary. 



And here I feel inclined to offer a few remarks on the theory 

 ,of magnetism, which are naturally suggested by tlie facts which 

 have been stated. 



The great similarity that subsists amongst the general charac- 



