88 Standard Measure of the Magnetic Intensity. 



of the terrestrial magnetism on the needle will undergo some 

 corresponding change, which will bear a constant ratio to the 

 change in the deflecting forces, the extent of which is known. 



If the forces of deflection and of the terrestrial magnetism 

 should both change, then, from what has been said above, it 

 appears that the relative amount of each can be estimated. 



In the foregoing examples, I have assumed that the deflec- 

 tor and needle suffer equal changes of intensity; and this, I 

 think, they will do (if they change at all), after some time, 

 when they have found their level : but it is well to provide for 

 any contingency ; therefore, to detect any alteration which 

 may take place in their relative forces, it is only necessary to 

 substitute the deflecting magnets for the needle, and to allow 

 the latter to deflect the former; and this can be accomplished 

 by inserting the ends of the magnets in two sockets, opposite 

 to each other, and furnished with a transverse axis like that 

 of the needle, so that when balanced on the jewels, the com- 

 bined magnets may become a dipping-needle, and the amount 

 of its deflection, caused by the needle when properly adjusted 

 in connexion with the verniers at the back, will indicate any 

 change in its force, if such should have occurred. 



It may be remarked, as it respects these combined magnets, 

 as well as the dipping-needle itself, that any error in their 

 construction will be always constant when their position is 

 horizontal, and that therefore it will not affect the value of 

 their indications. 



It is now more than a year and a half since Watkins and 

 Hill, No. 5, Charing Cross, made my first dipping-needle 

 with a deflector; and I showed it to several scientific indivi- 

 duals in London, and also at Oxford, when I a! tended the 

 meeting of the British Association in that city. I afterwards 

 had a larger one made in Cornwall, and Watkins and Co. 

 are now constructing another, in which I intend to have some 

 improvements made in the method, above described, of at- 

 taching the deflector, &c, to the back of the instrument. 



It is evident that needles, balanced with knife-edges resting 

 on agates, may be used in the manner before mentioned, for 

 obtaining a permanent standard measure of the magnetic in- 

 tensity ; but although this method may be preferred by many 

 to the chronometer one, for local use, I think that it will not 

 be deemed so eligible for locomotive purposes. 



