SS Mr Watt (y)i a New Magnetical Instrument^ 



to the influence that may affect it, always indicates a greater de- 

 gree of disturbance during the first experiments that are made 

 upon it, than it does after being subjected to a repetition of 

 them. In a few days, therefore, this instrument ceased to re- 

 volve for such a length of time ; and after being exposed to the 

 solar rays for five or six days, only revolved for a few minutes, 

 when first it met their influence, but continued to remain sta- 

 tionary, while the pivot moved with the earth, as long as the 

 sun remained unclouded. Towards the sun's meridian altitude, 

 however, when the thermometer was high (about 70°), as if sur- 

 charged by the solar rays, it sometimes vacillated a little, or 

 turned quite round, but rested in its usual position ultimately. 

 I found this instrument extremely sensitive, quickly indicating 

 by its motion an increase of heat, light, electricity, galvanism, 

 or a change of colour in the light that shone upon it. It moved 

 readily to any electric, when very slightly rubbed and held near 

 one side of the circle of needles ; and to all the prismatic co- 

 lours, and particularly to the solar beam, when concentrated by 

 a lens, and passed through coloured glass or silk. The violet 

 and red ray had the greatest effect upon it. 



When a piece of scarlet dark-blue or purple coloured velvet, 

 of a circular shape, is placed over the face of the instrument, so 

 as to cover the needles, the sensibihty of the instrument is great- 

 ly increased. 



When first placed with a disk of purple-velvet across the 

 needles, in the sun''s rays, it continued to revolve nearly the 

 whole of the day, moving always in the direction from east to 

 west by south, in the course of the sun's apparent motion. It 

 was attracted by a piece of coal or wood, ignited to red heat. 



It moved also 40° or 50° to the light of a single candle, held 

 close to one side of the circle. All these experiments were 

 made when the instrument was inside the glass-bell, and the 

 bodies affecting it outside. 



The same phenomena were exhibited when the needles were 

 all placed inside of a circle, all their points nearly meeting in the 

 centre, and no part of the needle appearing outside the circum- 

 ference of the ring. When suspended, it turned always one 

 side of the circle to the sun *. 



* These experiments were made in May and June 1827. 



