404 



NA TURE 



\_Feb. 25, 1886 



error of only o'i4 per cent. The formula for finding m, the co- 

 efficient of viscosity of nir at the temperature t° C, is therefore — 



ixi = 000017296 ( I + 



273^ 



The value of the coefficient of viscosity of air at 0° C, given 

 above, though much nearer to that obtained by Maxwell than 

 any which has been got by other observers, nevertheless tlifters 

 from it by more than 8 p-r cent.' 



January 28. — "On Local Magnetic Disturbance in Islands 

 situated far from a Continent." By Staff-Gommander E. W. 

 Creak, R.N., F.R.S. 



It has long been known that local magnetic disturbance has 

 been found to exist to a considerable extent at St. Helena, 

 Bermuda, and other islands. Observers in the islands adjacent 

 to the west coast of Scotland have also found local disturbance 

 existing in them, and, in order to determine its amount, have 

 obtained normal values from curves of the magnetic elements 

 calculated from neighbouring regions where observations have 

 been made apparently free from magnetic disturbance. 



In the case of islands situated far from a continent, however, 

 normal values of the three magnetic elements may be obtained 

 by the method of turning the ship in azimuth and ob erving on 

 eight or more equidistant points of the compass in the process 

 called " swinging," whereby the effects of the horizontal disturb- 

 ing forces proceeding from the iron of the ship may be 

 eliminated, and by occasional observations at well-selected land 

 stations known to be free from local magnetic disturbance, the 

 values of the vertical magnetic forces caused by iron in the ship 

 may also be ascertained for all latitudes, and the necessary cor- 

 rections applied to the observations. 



A series of magnetic observations have been made on land in 

 Bermuda, Madeira, Teneriffe, St. Vincent (Cape de Verde), St. 

 Paul Rocks, and Sandwich Islands in the northern hemisphere 

 and Tristan d'Acunha, Ascension, St. Helena, Kerguelen 

 Island, New Zealand, and Juan Fernandez, in the southern 

 hemisphere. The observations made in these several islands 

 have been collected for this paper and compared with the 

 normal values as observed at sea in their neighbourhood. 



Throughout the discussion the term "blue" magnetism has 

 been adopted to indicate that kind of magnetism which attr.acts 

 the marked or north-seeking end of the needle, and "red '' for 

 that which repels it. 



At Bermuda the most extensive series of observations h.as 

 been made, and a strong focus of blue magnetism found to exist 

 between Mount Langton and the lighthouse on Gibbs Hill. 



The position of this focus was approximately defined by 

 drawing, on a map of the western portion of Bermuda, lines of 

 equal values of the disturbance from the normal for each ele- 

 ment, and it was found that at one position eastward of this 

 focus the westerly declination was increased 2° 39', and at 

 another, westward of it, diminished by 3° 5'. The disturbance 

 of the inclination and vertical force gradu.ally increases as the 

 focus is approached, amounting in the inclination from +0" 11' 

 to 1° 47'. and in the vertical force from zero to -I- '314 (British 

 units). 



Bermuda may be taken as an example of the results generally 

 found in the other islands under discussion, for the observations 

 show that, north of the magnetic equator, the north point of the 

 compass is invariably attracted inland towards some part of the 

 island, and south of the magnetic equator it is repellei, showing 

 marked divergence of results between observations made on the 

 east and west coasts. The inclination and verticil force are, 

 with rare exceptions, increased in the islands on both sides of 

 the magnetic equator. 



On the whole the local disturbances are not very large, but it 

 may be remarked that they render the comparison of observa- 

 tions at different epochs very doubtful in value, unless the precise 

 position of observation be rigidly adhered to. 



Before dismissing the question of the actu.al observation, the 

 results obtained .it the Bluff, Bluff Harbour, New Zealand, are 

 worthy of note — 



( On the sunrnit of the Bluff ... 6 54 E. 



Declination ) 30 feet north ,, ... 9 36 W. 



observed 1 ,, west ,, ... 5 4 E. 



( ,, ea-.t ,, ... 46 44 E. 



Normal from sea observations 16 20 E. 



' Prof. Stokes, in a note at the end of the paper, has shown that a very 

 small deviation from horizontality of the movable disks used by Maxwell 

 would make the value of the coefficient obtained by him 8 per cent, too 

 great. 



On the summit of the Bluff there was thus shown to be .\ 

 strong focus of red magnetism. 



The general results tend to show that the magnetic disturb- 

 ance in islands north of the magnetic equator is due to an excess 

 of blue magnetism, and in those south of it to .an excess of red 

 magnetism compared with that due to the respective positions 

 of the islands on the earth considered as a magnet. 



In Sir G. Airy's treatise on magnetism, reasons have been 

 given for believing that the magnetism of the earth is not due t'> 

 sources external to it, nor specially existing on its surface, bui 

 that the source of its magnetism lies deep. 



With these reasons in view, and the results obtained from the 

 observations discussed, the posible conclusion is drawn that 

 the excess of "blue" and "red" magnetism observed in the 

 islands .above-mentioned proceeds fro u portions of those islands 

 which have been raised to the earth's surface from the mag- 

 netised part of the earth, fo.vning the source of its magnetism. 



For the numerical data u on which the preceding remarks 

 have been founded, and descriptive map of the Bermuda mag- 

 netic disturbances, reference should be made to the original 

 paper. 



Linnean Society, Febra-a-y4. — Sir John Lubbock, Bart., 

 Tresident, in the chair. — Mr. J. Dallas, of the Exeter Museum, 

 exhibited a specimen of the somewhat rare glossy ibis {Plegadis 

 falcinellits, L.), obtained from Mr. J. H. Clyde, of Bradsworthy 

 Vicarage, Holsworthy, Devon, in whose possession it had been 

 since killed in that neighbourhood, September 1851. — Mr. F. J. 

 Hanbury showcl a series of forms of the genera Hieracium 

 and Carex gathered by him in Caithness and Sutherlandshire, 

 all new to Britain, hut representative of the Scandinavian flora. 

 — Mr. C. Bartlett showed a remarkable African (?) caterpillar, 

 7 inches long, of a steel-gray colour, and abundantly hairy and 

 spiny. — Mr. W. H. Beeby drew attention to an example of 

 Etjuuitum litiorak got by him on Bisley Common, Surrey, and 

 hitherto not known .as an English plant. — Mr. J. C. Savver ex- 

 hibited a sample of a superior .-ort of the essential oil of lavender 

 and a spike of the plant, a cross-breed of varieties introduced 

 by him from the Continent, and grown at Brighton. — Mr. A. 

 Hammond showed a microscopic section of the integument of 

 the larva of a dipterous insect (Stnitwiiiys c/iamir/eon), raising 

 the question as to whether the polygonal areas of the cuticle, 

 described by M. ViUiane-, were surface-markings only, or, as 

 he held, cellular in character. — .Mr. F. Darwin read a paper on 

 the rel.ation between the bloom of leaves and the distribution of 

 the stomata. " Bloom " on leaves is used by him to mean a 

 coaling of minute particles of a waxy character, which is re- 

 movable by hot water or ether. But gradations occur from a 

 distinct and appreciable greasiness throwing off moisture to such 

 as are easily wetted. A large series of leaves of different groups 

 of plants have been studied by him, and for convenience in the 

 analysis of data he has divided them into four classes. Leaves 

 of Clas- I. are devoid of bloom on both surfaces, and yield 54 

 per cent, which have fio stomata on the upper surface. In 

 Class II. bloom is deficient above bitt present below, whereof 

 83 per cent, contain stomata on the leaves' lower surface. 

 Class III. possesses bloom on the leaves above, but none 

 inferiorly, and 100 per cent, of these have stomata on the 

 upper surface. Class IV. have leaves with bloom on both 

 surfaces, 62 per cent, of them having stomata above. From 

 such analyses and other facts and data given, Mr. Darwin con- 

 cludes that the accumulation of stomata accompanies that of 

 bloom, and, other things being equal, that it is functionally 

 protective against undue wetting by rain, and thus injury to the 

 leaf-tissue. — In a communication by Mr. E. C. Bousfield, on 

 the Annelids Slavina and Ophidonais, he criticises Herr 

 Vejdovsky's new genus Slavina, and objects to his identification 

 of Nais affiendiculata and .V. liirii/a, while giving a full descrip- 

 tion of the latter, an 1 observing points of contrast. He also 

 describes touch organs in Ophidonais, similar to those of 

 Clavina, mentioning other points of similarity between the two. 

 He further proposes to do away with the former genus, including 

 its only species under Slavin.a. — Brigacle-.Surgeon E. Bon.ivio, in a 

 paper afterwards read, asserts that the wild Citrus liysin.\, D. C, 

 is the grandparent of Lima tuberosus, L. a<;rcstris, Limoiiis fert, 

 Limonclhts auritriiis, and others, while also more distantly the 

 grandparent of the cultiv.ated true limes of India, Ceylon, &c. 

 The reason why the lime h.as so persistently a winged petiole, 

 according to him, is that this is derivative from the immense 

 winged petiole of its progenitor Citrus hyJrix. — Prof. Richard 

 J, ,\nderson communicated a i^aper on the relative lengths of 



