Oscillations of the Horizontal Needle, 19 



rays ; and their views have received much confirmation from the 

 many important inquiries of Mr Christie, and which have at va- 

 rious times appeared in the Transactions of the Royal Society. 

 These researches are of much consequence to experiments with 

 the oscillating needle. If a magnet be caused to vibrate in 

 the shade in air, and subsequently when exposed to the rays of 

 the sun, the time in which the arc of vibration becomes reduced 

 to a given point will in the sunshine be considerably less, whilst 

 the rate of vibration seems upon the whole to be somewhat in- 

 creased. 



In briefly detailing the following experiments made with va- 

 rious bars, both in the shade and in sunshine, I would by no 

 means pretend to have decided the question, whether the solar 

 ray has really magnetic properties or not. My object is merely to 

 shew, that certain differences observed to arise in the arcs and 

 times in vibration, under a given solar influence, may be caused 

 to vanish by placing the needle in an exhausted receiver. 



32. On exposing an oscillating magnetic bar to the influence of 

 a bright sunshine, so as to compare the phenomena incidental to 

 the state of vibration with those previously observed in the 

 shade, all the effects so accurately noticed by Mr Christie be- 

 came strikingly apparent ; but whether the bar be caused to vi- 

 brate in a dense, or in a rare medium, the experiment, under 

 an air-tight glass, is quite unmanageable for any accurate pur- 

 pose. The sources of inconvenience appear to be, 1st, an al- 

 teration in the rare or dense medium surrounding the bar, to- 

 gether with a constant circulation of currents ; 2d, a condensa- 

 tion of vapour over the interior of the glass ; 3d, various mecha- 

 nical and other changes in the conditions of the suspended bar 

 itself. These circumstances rendered the experiments in a mid- 

 day sun so very precarious, that I was obliged to abandon them. 

 33. (d.) With a view of avoiding the above mentioned sources 

 of inconvenience, I eventually conducted the experiments in 

 the following manner. Having observed the oscillations in the 

 c 2 



