Mr Watt's Description of the Magnetometer. 377 



which are exhibited at different seasons of the year, and in dif- 

 ferent parts of the globe, the instrument here described, 

 though simple, sufficiently supplies, I conceive, that desidera- 

 tum. 



If two magnetic needles are placed on two pivots, alongside 

 each other, in the magnetic meridian, at any distance within 

 the sphere of their repulsive powers (say an inch), with similar 

 poles opposite to each other, it is evident that, whether any in- 

 crease or diminution of their repelling powers takes place or not, 

 it cannot be indicated, as it would affect all the four poles equal- 

 ly ; and, therefore, they would always maintain the same rela^ 

 live position. 



But if, instead of balancing two needles by their centres, we 

 take two small bars of wood, of three or four inches long, w w, 

 w w, Plate y. Fig. 9- ; insert a smooth capsule into each of 

 these pieces of wood at the distance of an inch from their 

 ends, so that an inch of the wood shall project beyond the 

 capsule ; and if, to these ends, we affix two light magnets, 

 m m, m m, three inches, for example, of the main-spring of a 

 watch made straight, and balance these on two pivots pp, in the 

 magnetic meridian, with similar poles opposite to each other, 

 they will then be free to move to any increase or diminution of 

 magnetic influence, and indicate, by their motion, any alteration 

 of the kind by their repelling properties accordingly ; for the 

 magnetic meridian acts, in this case, as a common point of at- 

 traction to both. When, therefore, they are placed at the dis- 

 tance of two or two and a-half inches from each other, they will 

 be repelled 20° or 25° off from the meridian, and if a scale of de- 

 grees is placed beneath the wooden extremities of the two bars, 

 a very great variation in the magnetic intensity takes place, and 

 can readily be observed. 



This instrument is formed upon the acknowledged laws, that 

 the repulsive powers of the magnet increase in the same ratio 

 with their attractive powers ; and that both increase or decrease 

 with the augmentation or diminution of the magnetic intensity, 

 as variously developed, in different places, on the surface of the 

 globe, and, as this instrument proves, at different seasons of the 

 year. 



When, therefore, the magnetic force increases, the magnets 

 are repelled from each other, and diverge farther from the me- 



