Chemical Science. 421 



place where the wire first touches the mercury, equivalent to 

 a diminution of the cohesive attraction of the mercury. The 

 efiect rapidly diminished by increasing the size of the wire, and 

 20 pair of plates of Dr. WoUaston's construction, and four 

 inches square, would not produce it with the fine wire : on the 

 contrary, two large plates are sufficient. Dr. Hare's calori- 

 motor was the instrument used, and the charge was so weak 

 that it would barely warm t»vo inches of any sized wire. 

 Whether the effect is an actual diminution of the attraction of 

 the particles of the mercury, or depends on some other cause, 

 remains as yet to be determined. But in any case its influence 

 is so powerful, that it must always be estimated in experiments 

 made to determine the force and direction of an electro-magnetic 

 wire, acted on by a magnetic pole, if the direction is otherwise 

 than horizontal, and if they are observed in the way described 

 in this note. Thus, at the magnetic equator, for instance, where 

 the apparent alteration of weight in an electro-magnetic wire 

 may be expected to be greatest, the diminution of weight in 

 its attempt to ascend would be increased by this effect, and the 

 apparently increased gravity produced by its attempt to de- 

 scend would be diminished, or perhaps entirely counteracted. 



I have received an account by letter from Paris, of au ingeni- 

 ous apparatus (see page 415,) contrived by M. Ampere, to il- 

 lustrate the rotatory motions described in my former paper. 

 M. Ampere states that, if made of sufficient size, it will rotate 

 by the magnetic action of the earth, and it is evident that that 

 will be the case in latitudes at some distance from the equator, 

 if the rotatory wires, namely, these by which the ring of zinc is 

 suspended, are in such a position as to form an angle with a 

 vertical line, larger than that formed by the direction of the 

 dip. 



It is to be remarked, that the motions mentioned in this 

 note were produced by a single pair of plates, and therefore, 

 as well as those described in the paper, page 74, are the re- 

 verse of what would be produced by twoor more pair of plates. 

 It should be remembered also, that the north pole of the earth 

 is opposite in its powers to what I have called the north poles 

 of needles or magnets, and similar to their south poles. 



I may be allowed, in conclusion, to express a hope that the 

 law I have ventured to announce, respecting the directions of 

 the rotatory motions of an electro-magnetic wire, influenced by 

 terrestrial magnetism, will be put to the test in different lati- 

 tudes ; or, what is nearly the same thing, that the law laid 

 down by M. Ampere, as regulating the position taken by his 

 curve, namely, that it moves into a plane perpendicular to the 

 dipping-needle, will be cxperimcntuUy ascertained by all those 

 having the opportunity. 



