Feb. 2, 1888] 



NA TURK 



321 



The Temporary Thermo-Current in Iron. 



In the Pkilosopliical Magazine for January, Mr. Herbert 

 TomUason has proposed an explanation of the remarkable fact 

 that in an iron wire, heated red hot by a burner, an electric 

 current is produced when the flame is shifted along the wire 

 (see Wiedemann's " Galvanismus," ii. 453). 



As his explanation is inadequate, perhaps I may be excused 

 again drawing attention to this subject. Briefly his explanation 

 is as follows : — That, as the portion of the wiie in the flame rises 

 in temperature, it, thermo-electrically speaking, becomes in fact 

 like a different metal, and that then, on shifting the flame, the 

 junction with the unaltered wire on the side moved towards 

 becomes hotter than before, while the one on the other side 

 falls in temperature, thus presenting the ordinary case of a 

 thermo-couple with junctions at different temperatures. Now 

 this explanation entirely overlooks the fact that, by the first 

 assumption, just as fast as the temperature on one side rises, the 

 wire there changes into tlie "second state," and corre- 

 spondingly changes back on the other side as the tempera- 

 ture falls there ; so that, as far as this explanation goes, there 

 ought to be no current whatever, for thus both junctions 

 must always be at the same temperature. 



When I first noticed this current, which from considerations 

 to follow I have ventured to call the "temporary thermo- 

 current," it appeared to me to be due to the difference in the 

 temperature-slope (or gradient) along the wire in front from that 

 behind the flame, as it heats more rapidly in front than it cools 

 behind, and to the electromotive force being a function of the 



la 



slope, i.e. of ^^. But this hypothesis did not stand the test of 



ax 

 experiment, as I have shown in a paper published in 

 the Proceedings of the Royal Dublin Society, July 1886. 

 So that as there is a current, we must suppose the 

 "second state" to be not only a function of the tempera- 

 ture, but also of the time, i.e. that the wire changes into 

 (or from) the "second state" more slowly than it is possible 

 for it to change in temperature. So that the electromotive force 

 at any point depends on the rate of change of the temperature 



slope, or equals <p ( 't '— ]. In support of this it will be found 

 \<U ax) 



that if the flame he steadily moved along very slowly no current 

 is produced — at all events less than would be otherwise ex])ected ; 

 and, secondly, that the maximum current is got by moving the 

 flame the fastest consistent with the condition of keeping the 

 wire red hot. 



It is with the view of emphasizing this dependence on the 

 time that the term "temporary thermo-current " seems appro- 

 priate. Fred. T. Trouton. 



Physical Laboratory, Trinity College, Dublin. 



Causes influencing the Bathymetrical Range of Deep- 

 Sea Fishes. 



You refer (p. 219) to the fact that Dr. Giinther has 

 adopted the lOO-fathom line as the boundary at which 

 with the extinction of sunlight the bathybial fauna com- 

 mences. This selection of 100 fathoms as the limiting 

 horizon is of much interest in connection with the theory 

 that the shallow-water marine fauna is greatly influenced by 

 wave-currents. In a letter you published in 1885 (Nature, 

 vol. xxxii. p. 390) I indicated 100 fathoms as the depth to which 

 wave-action nf some sort must extend, as evidenced by the 

 character of the deposits at the mouth of the English Channel. 

 Dr. Giinther now shows that the deep-sea fishes do not rise 

 above that horizon. But, although the loo-fathom horizon 

 agrees very well with the apparent limit of wave-action, it does 

 not seem to agree with the most recent experiments on the pene- 

 tration of sunlight in water. 



So recently as November last you recorded the fact that 

 during the pait year Prof. Forel found that the greatest 

 "depth-limit of absolute darkness" from March to July in the 

 Lake of Geneva was 100 metres (Nature, vol. xxxvii. p. 88). 

 If experiments in a fresh-water lake may be taken as a guide to 

 light-penetration in the ocean, 50 fathoms will be nearer the 

 limit than too. In this case the bathymetrical range of the 

 bathybial fauna cannot be much influenced, if at all, by the 

 presence or absence of sunlight. This view is moreover fortified 

 by the fact that, though the deep-sea forms do not usually ascend 



above the loo-fathom line, the shallow-water forms go far below 

 it ; and there is no reason why they should not do so ; for, 

 although a form unfitted to withstand wave-currents cannot face 

 them, there is nothing to prevent a flat-fish, fully equipped in 

 this respect, from passing at will from the disturbed to the 

 tranquil horizon, and vice versa. A. R. Hunt. 



Torquay, January 10. 



Wind Force at Sea. 



In reference to a letter on the above subject in Nature 

 (P- 274), I beg to acquaint your readers that Capt. Barker's 

 wish that anemometers should be used more on board ship has 

 been endeavoured to be met by an instrument designed by 

 myself on the sail principle. It has now been in use on some 

 ships at sea for long voyages for five years, and daily observa- 

 tions have been obtamed and sent home of the data observed, of 

 pressure, direction, and velocity of the winds met with. 



Regardingthefurtherinquiry of ascertaining the rainfall at sea, 

 this has now been carried on for about ten years by means of a 

 rain-gauge designed by my>elf on the pivot principle, and it has 

 been used by many vessels in all the great seas. 



The daily observations have been sent home and are now on 

 hand, and about five years of the returns have already been 

 announced, and a further compilation of the data may be prepared 

 when the materials become sufficient. 



It may be added that the late Capt. Symington, of the s.s, 

 Hankcnv, amongst his meteorological observations took the 

 rainfall by rain-gauge on his ship for twenty years or more. 

 The marine anemometer and rain-gauge at>ove mentioned were 

 exhibited at the Liverpool Exhibition in 1886, and at the 

 Meteorological Exhibition of last year. W. G. Black. 



Edinburgh, January 21. 



Untimely Insect Development. 



Some of your readers may be interested in a case of untimely 

 insect development, caused no doubt by the phenomenal mild- 

 ness of the weather in this part of the country during the last 

 few days. Last evening a perfect imago of the common tortoise- 

 shell butterfly (Vanessa urtica) was found inside my house on the 

 wall of my nursery. It is. fully developed in every way, and the 

 only thing in its appearance at all abnormal is that the antennae 

 are bent back and he between the wings, which are in the erect 

 position usual in repose. The insect has evidently only just 

 emerged from the pupa, and is in a torfad condition, only just 

 flapping its wings when touched. The nursery is a warm room 

 looking to the south, and has a fire in it all day. 



St. Albans, January 10. John Morison. 



Weasels killing Frogs. 



Seeing a note in Nature (December 29, 1887, p. 208), about 

 weasels killing frogs, I thought that the following fact would be 

 a further confirmation. 



I was walking near the village of Clifton Hampden in August 

 last, when I saw a weasel, carrying a good-sized frog in its mouth, 

 come cautiously out of the rank grass by the road-side ; directly 

 the weasel perceived me, it dropped its prey on the road and 

 retreated to the cover of the grass. The frog was dead. I kept 

 silence, and the weasel left its hiding-place, and advanced a few 

 steps, but again retreated. Soon, after several advances and 

 retreats, it rushed out, seized the frog with its teeth, and running 

 across the road disappeared in the long grass on the other side. 



January 20. M. S. Pembrey. 



"British and Irish Salmonidae." 



The author of "British and Irish Salmoniure " calls in 

 question the justice of three criticisms in my review of that 

 book. In reply to his first objection, I have to point out that 

 my quotation of the sentence referred to was, as Mr. Day has 

 himself noted, made to draw attention to its grammatical errors, 

 and therefore the omission of a few words which affected the 

 sense but not the construction was of no consequence at all. I 

 omitted ihe words intentionally, to shorten the quotation, and 

 gave no opinion on the statement contained in the sentence : the 

 statement which is implied rather than expressed is perfectly 

 correct. 



