July 8, 1875] 



NATURE 



187 



it may be supposed that the laws of disturbance are not found 

 by me, and are not to be found by the method which I have 

 employed. This would be a great mistake ; one which I am 

 bound to correct. 



The method which Prof. Stewart recommends has had objec- 

 tions proposed to it by the Astronomer Royal, the Provost 

 of Trinity College, Dublin, and by myself. It is, I think, to 

 defend the method against these objections that Prof. Stewart 

 has written his remarks on the modes of discussion ; but I have 

 never heard any objections to the other method, nor, as far as I 

 can understand, does he offer any. 



As the method followed by Dr. Lloyd on the Dublin Obser- 

 vations and by myself on the Makerstoun and Trcvandrum 

 Observations has shown every law of magnetic disturbance that 

 has been obtained by the other, I am afraid I cannot see that 

 the illustration of the cyclones is applicable to the two methods, 

 even if we were bound to study large cyclones only and to put 

 those of less than a given magnitude out of consideration. 



4, Abercom Place, London, N.W. John Allan Broun 



Anomalous Behaviour of Selenium 



It has been lately observed that the electrical resistance of 

 selenium is greater in the light than in the dark. It was at first 

 thought possible that this increase of resistance might be due to 

 heat admitted with the light, but Prof. W. G. Adams, in his 

 paper read before the Royal Society, June 17th, 1875, ^^s 

 shown that this is not the case, but that the phenomenon is a 

 purely optical one. 



The writer of this letter has to-day tried an experiment with a 

 selenium bar belonging to the Cavendish Laboratory. Its length 

 is 50 mm., breadth 8 mm., thickness about I mm. ; platinum 

 wires are soldered to its ends, and it has a hard metallic surface. 

 Its electrical resistance is enormous. In the dark it is just over 

 100 megohms (100,000,000 B.A.U.) When, however, the light of 

 the paraffin lamp of the galvanometer was allowed to fall, on it 

 from the distance of about a foot, the resistance decreased 

 between 20 and 30 per cent. The experiment was repeated 

 many times, with current sent sometimes one way, sometimes 

 another, and with different sides and edges of the bar turned to 

 the light, but always with the same result, namely, that the effect 

 of letting in the light was to largely decrease the resistance, 



A second set of experiments were made with a selenium medal 

 struck by Berzelius soon after the discovery of the metal in 1818, 

 and presented by him to Mr. Deck, by whose son, Mr. Deck of 

 Cambridge, it was kindly lent for the experiment. This medal 

 was of oval shape, about 40 millims. long by 30 broad. Owing 

 to the difference of form between the two specimens, their 

 specific resistances could not be accurately compared ; that of 

 the medal was, however, not more than about ^V of that of the 

 bar. The medal was exactly like black lead both to touch and 

 sight, and quite different in appearance to the bar. The resist- 

 ance of the medal ivas sensibly the same, both in the dark and 

 in the light ; no difference could be detected. 



These experiments seem to show that the physical form of 

 the metal has a great deal to do with its behaviour when carrying 

 an electric current and exposed to light. 



J. E. H. Gordon 



Cavendish Laboratory, Cambridge, June 29 



The House-fly 



As no one more competent than myself seems disposed to 

 reply to the query of " Harrovian" (Nature, vol. xii. p. 126) 

 respecting a disease of the house-fly, and which is again referred 

 to by the Rev. D. Edwardes in last week's Nature, I may 

 perhaps be permitted to make a few remarks thereon. 



I have frequently noticed dead and dying flies thus affected, 

 generally in the late summer and autumn ; and I think I am 

 right in attributing the phenomenon to the growth of a parasitic 

 fungus, called, I believe, Evipusa mtisci, in the fly's body. The 

 insects may otten be seen settled in a natural position on window- 

 panes, but with the abdomen much distended, and surrounded 

 by a collection of whitish powder, extending for a few lines in 

 all directions on the surface of the glass. The whole of the 

 interior organs of the abdomen are consumed by the plant, 

 nothing remaining but the chitinous envelope, on which the 

 mycelia of the fungi form a felt-like layer ; the fructification 

 showing itself externally as filaments protruding from between 

 the rings of the body. 



Insects are very subject to the attacksof such parasites. Some 

 of those living in the interior organs of the body seem to do little 

 if any injury to their " hosts," while others completely destroy 

 them ; as in the case of Sph(eria,\'wh\ch. changes the caterpillars 

 at whose expense it lives into a mass of fungoid growth of most 

 grotesque appearance. It is now well ascertained that a species 

 of Botrytus produces the dreaded "Muscardine" of the silk- 

 growers ; and every practical lepidopterist has had to lament 

 the destruction of pet broods of larvae by some similar disease, 

 which, though perhaps sometimes pathological, is probably in 

 the first instance set on foot by fungi. 



The whole subject of the parasites of insects is extremely in- 

 teresting. According to my experience it is the exception for 

 an insect to be quite exempt from the attacks of one or more 

 animal or vegetable entozoic or epizoic organism ; and I have 

 often found five or six different species inhabiting one unfor- 

 tunate individual. 



I may mention that " Harrovian " will find some remarks on 

 this fly-fungus by Dr. Cohn, in an early volume of the Journ. 

 Micros. Science. I regret that, writing away from home, I 

 cannot give the exact reference. W. Cole 



Stoke Newington, N., July 2 



[We print this letter from among several which all correctly 

 explain the phenomenon under consideration in a similar 

 manner. — Ed.] 



Theories of Cyclones 



In Nature, vol. xii. p. 98, you notice a paper by Dr. Hann 

 on two rival theories of cyclones. According to one, "whirl- 

 winds are formed mechanically by different streams of air meet- 

 ing, and centrifugal force causes the central depression. The 

 more modern theory regards a local depression as the first con- 

 dition, causing an indraft resulting in a whirlwind through the 

 earth's rotation : the primary depression is held to follow con- 

 densation of vapour." 



The question is how the cyclone begins : whether the first 

 depression is due to the centrifugal force of an eddy, or to the 

 expansion of air in the upper strata from the heat liberated in 

 the condensation of vapour. There need not be any controversy 

 as to the dynamics of the cyclone after it is formed. 



There is a mass of geographical evidence in favour of the first- 

 named theory, namely, that cyclones originate in the conflict of 

 the trade-winds of the northern and southern hemispheres when 

 either trade-wind is drawn to some distance across the equator. 

 (A cyclone cannot be formed on the equator, because there the 

 earth has no rotation in relation to a vertical axis). On this 

 subject see Mr. Meldrum's paper in Nature, vol. ii. p. 151, 

 and my letter in Nature, vol. iv. p. 305 ; also Mr. Maury's 

 paper in Nature, vol. viii. pp. 124, 147, 164. 



Mr. Maury fully recognises the truth that the motive power of 

 the cyclone, once it is formed, consists in the heat liberated by 

 the condensation of vapour, which causes expansion in the upper 

 strata and produces an ascending current. 1 believe the nature 

 of these actions was first explained by Espy, whose " Philosophy 

 of Storms," though well known by name, seems to be less appre- 

 ciated than it deserves. 



There is, however, another reason for the existence of an 

 ascending current at the centre of a whirlwind, which I do not 

 think I have seen stated. The lowest atmospheric stratum of a 

 whirlwind is retarded in its motion by friction against the earth, 

 and its centrifugal force is thereby lessened in proportion to that 

 of the upper strata. The effect of this relative deficiency of cen- 

 trifugal force in the lowest stratum — that is to say, at the surface 

 of the earth — must be to cause a flow of air at the surface of the 

 earth towards the centre of the whirlwind, and an ascendmg 

 current at its centre. Such an ascending current is probably the 

 cause of the vertical columns of dust that accompany those small 

 whirlwinds which are common in windy weather. 



Old Forge, Dunmurry, Joseph John Murphy 



Co. Antrim, June 23 



The Dark Argus Butterfly 



It is stated in H. N. Humphrey's work on " British Butter- 

 flies," that the Dark Argus Butterfly appears in July, and has 

 only been found in the neighbourhood of Durham and New- 

 castle, and seldom above half a mile from the sea. When in May 

 I was at Ashmore, which is on the borders of Dorset and Wilts, 

 I took some butterflies answering exactly to the description of 



