382 Mr Watt'*s Experiment with the Magnetic Needle* 



influence in the equatorial regions prevents the needle from, 

 either varying or dipping much. Its being equidistant from 

 both poles of the earth may also be a farther cause. But in 

 the higher northern latitudes, the south pole of the needle is 

 apparently attracted to, and affected by, the sun ; whilst the op- 

 posite takes place near the antarctic regions, as far as we can 

 learn, the south pole then dipping, and the north being most af- 

 fected by the constant changes of solar influence. And it must 

 be owing to some celestial influence that a bar of steel, such as 

 has been described, when made perfectly straight and equally 

 balanced on all sides, stands quite perpendicular when rendered 

 magnetic, though before resting horizontally. 



I perceive that Professor Hansteen has also been making ob- 

 servations on the different degrees of magnetic intensity that 

 subsist during the seasons of the year ; and though I have not 

 heard by what means he has come to his conclusions, yet I un- 

 derstand they are quite the reverse of those I have formed, viz. 

 that there exists a greater degree of magnetic intensity during 

 winter than in summer, and yet I believe we may both be per- 

 fectly correct ; for my experiments regard chiefly the solar influ- 

 ence, the two opposing magnets being balanced quite horizon- 

 tally, while Hansteen's may have been made in respect only to 

 the terrestrial influence ; and the latter seems to increase as the 

 former diminishes, and vice versa ; but a certain proportion of 

 both appears necessary to produce the directive property of the 

 needle, as that property is lost near the north and south poles. 

 The terrestrial magnetism, indeed, most correctly ascertained by 

 the dip ; and I mentioned in my paper on the solar compass that 

 in this also there was a small diurnal and annual variation corre- 

 sponding to the sun's altitude and state of the atmosphere. 



Hansteen indeed ascribes the different degrees of magnetic in- 

 tensity which are developed in the successive months of the 

 year to the solar influence, and supposes that the magnetic in- 

 tensity is greater in winter than in summer, because the earth is 

 then in its perihelion ; but though this is certain in respect to the 

 sun, yet it is no less so, that the influence of the sun in our hemi- 

 sphere is immensely greater, and its force more powerfully felt by 

 every substance, organic and inorganic, in summer than in win- 

 ter, though the earth is then in aphelion. I have only to add 

 that my observations were made before I heard of Hanstcen's. 



