598 



NA TURE 



lApr/l 22, 1886 



the mean of 40S and 740) the effect on the dimensions of the 

 wire would cease altogether in the limits of the electric cur- 

 rents employed in the above experiments." 



In reference to this surmise the author in his paper of last 

 year expressed his belief that, if Joule had actually made the 

 experiment, he would have found that the length of the wire 

 was increased by a weak current, that a current of medium 

 strength would have had no effect whatever, and that one of his 

 stronger currents would have caused the wire to retract. He 

 had, in fact, reason to believe that the effect of tension was to 

 diminish the "critical magnetising force" (which produces 





K g 



a e 



a S 



s 



maximum elongation), so that the retraction which is found to 

 occur in all iron rods when a sufficient magnetising force is 

 employed, is observed with smaller magnetising currents when 

 the rod is stretched than when it is free, but want of suitable 

 apjiaratus prevented him from submitting this idea to the test of 

 direct experiment. 



He has lately modi6ed the instrument, which was described 

 in his former paper, in such a manner that it can be used for 

 observing the effects of magnetisation upon rods and wires under 

 traction, and the results of a series of experiments made with it are 

 presented in a synoptical form in the above table. Four speci- | 



mens of iron were used. The first was a wire of commercial iron, 

 I -2 mm. in diameter, which had been softened by heating in a 

 gas flame ; the second was a strip of annealed charcoal iron, 

 5"5 mm. wide and 0'55 mm. thick, its sectional area being about 

 3 mm. ; the third was a piece of hard unannealed wire, 2'6mm. 

 in diameter ; and the la^t was a wire of very pure soft iron, 

 3 '25 mm. in diameter, which had been carefully annealed. 

 These were successively fixed in the apparatus, and loaded with 

 weights varying from 3 lbs. to 14 lbs. While under the influ- 

 ence of each load, four observations were made in the case of 

 each wire : — (i) A determination was attempted of the smallest 

 magnetising current which sensibly affected the length of the 

 wire in the direction of elongation or retraction. (2) The 

 current prodncing maximum elongation (if any) and the extent 

 of such maximum elongation were found. (3) A determina- 

 tion was made of the critical current which was without effect 

 upon the original length of the wire, i.e. the current of such 

 strength that a weaker one would cause elongation and a 

 stronger one retraction. (4) The retraction produced by a fixed 

 current of I '6 ampere was measured. 



The figures recorded in the table disclose the following 

 facts : — 



(i) The effects produced by magnetisation upon the length 

 of an iron wire stretched by a weight are in general of the same 

 character as those which have been shown in the former paper 

 to occur in the case of a free iron rod. Under the influence of 

 a gradually increasing ma'^netising force such a wire is at first 

 elongated (unless the load be very great), then it returns to its 

 original length, and finally it contracts. 



(2) The maximum elongation diminishes as the load increases 

 according to a law which seems to vary wiih different qualities 

 of iron. If the ratio of the weight to the sectional area of the 

 wire exceeds a certain limit, the maximum elongation (if any) is 

 so small that the instrument fails to detect it. 



(3) The retraction due to a given magnetising force is greater 

 with heavy than with light loads. 



(4) Both maximum elongation and neutrality [i.e. absence of 

 both elongation and retraction) occur with smaller magnetising 

 currents when the load is heavy than when it is light ; retrac- 

 tion, therefore, begins at an earlier stage. Thus the anticipa- 

 tion expressed in the author's former paper is justified. 



(5) The phenomena, both of elongation and of retraction 

 are, as might be expected, greater for thin than for thick wires, 

 and for soft than for hard iron. 



Linnean Society, April 15. — W. T. Thiselton Dyer, 

 C.M.G., Vice-President, in the chair. — The following gentlemen 

 were nominated auditors, viz., J. Jenner Weir and F. Victor 

 Dickins, as representing the Fellows, and Thos. Christy and 

 F. B. Forbes for the Council ; afterwards Mr. Rochfort 

 Connor w.as elected a Fellow of the Society. — Specimens of 

 so-called Madrepore marble from Iowa (U.S.) were exhibited 

 for Mr. G. A. Treadwell. These contained abundance of a 

 species of Stromatopora. — Mr. E. A. Heath showed living 

 examples of D.'ndrobium dmsifloriim and D. sunTissimum, and 

 Mr. J. G. Baker drawings of new and remarkable ferns in illus- 

 tration of the Roraima report. — A paper was read on new 

 African genera and species of Curculionidse by Mr. F. P. 

 Pascoe. These were obtained from Momboia, a missionary 

 station north of Lake Nyassa, from Landana, a new settlement 

 on the Congo, and Mayotte, one of the Comoro Islands off 

 Madagascar, The author remarks that the inadequate descrip- 

 tions, without reference to affinities or diagnostic characters as 

 given by some entomologists, ought to be disap|)roved. The 

 great diversity of appearance among the same genus of Curcu- 

 lionidse is somewhat remarkable ; secondary characters, there- 

 fore, have to be taken into account, but these, after all, may be 

 quite as natural. On the other hand, species quite like each 

 other in appearance are found to belong to v\ idely different 

 groups. For these and other reasons the correlation of stable 

 characters is perplexing, and definite classification difficult. — The 

 third part of Mr. C. E. Broome's series of fungi from Brisbane, 

 Queensland, was read in abstract. — Mr. Everard F. im Thurn 

 then gave the gist of a long report on the plants collected by him 

 during his recent ascent of Mount Roraima, British Guiana. 

 Among these, 3 new genera and 54 new species had been deter- 

 mined. The country of Guiana was described by him as con- 

 sisting of three marked ascents from the Atlantic on the east to 

 the central table-land west. The groups of vast sandstone 

 columns, of which Roraima is the best known, really abut or • 

 overlap on to Brazil territory, and from their summit pour down 



