438 Sir G. C. Haughton on the Common Nature of 



nomena, and to which we are indebted for many very valuable 

 discoveries. That which I am now about to explain differs 

 from all that have preceded it, and possesses at least the ad- 

 vantage of simplicity united to singular delicacy*. 



It must have been clear to every one, that the phjenomena 

 observed by M. Arago in his well-known experiments made 

 in the year 1824, when he caused a needle to oscillate over 

 the surfaces of various substances, were due to the proximity 

 of the needle to the body observed, as much as to the nature 

 of the body itself; and the discovery of this distinguished 

 natural philosopher was confirmed by Dr. Seebeck of Berlin, 

 on making similar observations. Those I have now to detail 

 are entirely due to the same cause, carried to the extent of 

 actual a.x\d forced contact. 



I now proceed to explain the mode in which the experi- 

 ments have been performed. 



Mode of operating. 



If the substance to be examined is flat, as a piece of glass, 

 for instance, it should be set up on a very steady table, in a 

 vertical position, and a magnetic needle, of which a descrip- 

 tion will be presently given, is to be placed on the point of a 

 fine sewing-needle, about half an inch in height, which may 

 be run through a slice of cork to answer as a support ; but 

 that which I myself employ has a flat brass foot about the size 

 of a shilling, with the fine end of a sewing-needle fixed in the 

 centre, and the brass foot is truncated close to the supporting 

 needle, to enable it and the object to be brought close together 

 for the purpose of observation. If a cork support is used, the 

 sewing-needle should for the same reason be run through it 

 near the edge. 



When a minute object, such as a precious stone or any 

 small fragment of metal, is to be examined, it should be placed 

 on a piece of cork or any other suitable body, in such a posi- 

 tion to the natural direction of the needle, that on the latter 



* Tlie general principle upon wliich tiiese observations have been made 

 was iliscovercJ so fur back as the month of March 1841 ; it was not how- 

 ever applied to ascertaiiiing whether the various classes of substances were 

 magnetic or not till the spring of 1845, when nearly 140 substances were 

 found to be magnetic, anil a few measurements of the intensities were ob- 

 tained, but it was at the commencement of 184(5 that the greatest part of 

 them were made. The remainder iiave been only recently completed ; 

 though these remarks were in such a state of forwardness as to lead to a 

 hopeof their being published in the summer of 1845. Want of health has 

 been the cause of these various delays. 



