534 



NATURE 



[September 26, 1895 



Colonel Cunningham read a paper on Mersenne's numbers, 

 which are numbers of the form 2'- I, where </ is a prime, and 

 which were first discussed by Mersenne about the year 1664. 

 Colonel Cunningham also described a book of tables which he 

 proposed to calculate, giving the s<ilution of the congruence 

 2* = R (mod. p) for all mo<luli (/) which are primes, or powers 

 of primes, up to 1000. There are to be two tables for each 

 modulus, one giving the values of R for a series of values of x ; 

 and the other giving the smallest values of .v for a seiies of 

 values of R. I le described some of the uses of such a table, and 

 stated that the plan on which it would Ik; drawn up would be 

 precisely like a somewhat similar table by Jacobi, described in 

 Prof. Cayley's re|X)rt on mathematical tables in the British 

 Association Report of 1876. 



Prof. Alfred Lodge drew the attention of the Section to a 

 multiplication table up to 1000 x 1000, drawn up by Mr. M. 

 B. Cotiworth, of liiildgate, York, which w.-is exhibited ; it is 

 similar to Crelle's table of the same e.vtent, though in some 

 respects more convenient. 



Prof. M. J. M. Hill described two species of tetrahedron, the 

 volume of any member of which can be determined without 

 using the proposition that tetrahedrons on ef)ual bases, and 

 having etjual altitudes, are equal. 



In the department of meteorology, Mr. Eric S. Bruce put 

 forward a new theory of lightning flashes, based on the prin- 

 ciple <if the pin-hole camera. The light from a concealed flash 

 might, he supposed, pass through a small aperture in the con- 

 cealing cloud and fall on another cloud, forming an inverted 

 im.ige of the flash. If there were several apertures we should 

 have .IS many images. They would be faint, possibly too faint 

 to affect a photographic plate. Moreover, if the receiving cloud 

 were of irregular shajx', an originally straight flash would 

 apjjear distorted into a zig-zag line on the cloud. Mr. Symons 

 thought a brighter patch of light ought to occur at the angles of 

 the image thus distorted, and he scarcely thought the conditions 

 imagined by Mr. Bruce corresponded with those of nature. 



The report of the Committee on earth tremors was presented 

 by -Mr. Symons, who, in referring to the delicacy of the instru- 

 ments used in their observations, said that an angle equal to 

 that .subtended by a chord I inch long at the centre of a circle 

 1000 miles in radius could be detected. .Since last report two 

 bifilar pendulums have been purchased, of the kind described in 

 NatI'RE, vol. I. pp. 246-249 (1894); each possesses its own 

 photographic recording apparatus. One of these has been 

 recently erected in the cellar of Mr. Davison's house in Birming- 

 ham ; the other should have been placed in a house three- 

 quarters of a mile to the e.tst, but this was found impr.icticable. 

 It will lie placed somewhere in the neighbourhood, ami comjiari- 

 sons of the records of the instruments will be made during the 

 year, after which the second one will be available for another 

 • .•: n. .\n appendix to the report by .Mr. Davison gives the 

 1;' 1, ;;raphy and classification of horizontal pendulums. 



I'rof. John Milne gave an account of the lung report of the 

 Committee on .seism0logic.1l phenomena in Japan. This com- 

 mences by a reference to the great loss caused by the recent fire 

 at Prof. Milne's house .and observatory, after which follows a 

 description of the records of the Gray- .Milne seismograph. At- 

 tached to the re|H)rt is a catalogue of S331 earthquake shocks 

 recor<led in Japan between 1885 and 1892, giving full particulars 

 of the centre and area of disturlnnce. It enables the approxi- 

 mate Tit /^V'/ of each to be found, and permits the division of 

 Ja|Kin into fifteen distinct .seismic di.stricts. The next section of 

 the repirt deals with the rate of pro|iagation of earthquake dis- 

 turbances from Ja|>an to Europe. The small tremors which 

 occur in the ten seconds or so before an earthquake shock are 

 transmitted to Euro(>e, but they arc .spread over half an hour ; it 

 appears, therefore, that the preliminary tremors either travel 

 more quickly, or reach Kuro|«: by a .shorter route than the main 

 ' ' ' ' "ir is known to travel along the surface at about 



I second. Do the tremors travel at 8000 to 

 I , r second, or do they [kiss through the earth, 



not round it .' If the latter, we may hope for some further 

 kn'iwlfdg'- '■on'-i-ming the interior of the globe. Prof. Milne 

 ■ ndulums in nearly a score of places, and 

 in their liehaviour. They all exhibit a 

 ■.r. till, in the .same direction, and similar 

 Examined from hour to hour, how- 

 :.» the existence of a diurnal wave. After 

 ! very latxirious search, graphically descrilied lo Ihe 

 ■, Prof. Milne, he succeciled in tracing this diurnal 



NO. 1352, VOL. 52] 



effect to the local removal of load from the alluvium by greater 

 evaporation from exposed areas. At night the movement is 

 slight, and is probably accounted for by the condensation, at the 

 cold surface, of aqueous vapour after rising through the warm 

 earth. Some observations have been made on the disturbance 

 of the pendulums by earth tremors. Their cause has not been 

 asccitained, but they always occur with greatest intensity be- 

 tween 5 and 9 a.m. They are most marked with a steep liaro- 

 meter gradient and consequent wind, local or distant. 



As Prof. Milne has now relurne<l from Japan, and the earth- 

 quake catalogue is completed, the Committees on earth tremors 

 and seismological phenomena have united under the latter name. 

 The new Committee is a large one, and with Prof. Milne and 

 Mr. Davison as joint secretaries, it ought to do good work. 



A new theory of thunderstorms was advanced by Prof. Michie 

 Smith in his paper on Indian thunderstorms. His observations, 

 made at Madnas, showed that sheet-lightnins; occurs there every 

 evening during several months of the year, always in the south- 

 west and near the horizon. Lightning phenomena in the morn- 

 ing occur, on the other hand, in the north-east. The phenomena 

 consist of actual discharges between two clouds, or two jiortions 

 of the same cloud, and are not reflections of distant lightning ; 

 they take place in the upper portions of low-lying cunuilus 

 clouds. Prof. Smith attributes them to the clouds formed in 

 the regions of still air at the meeting of the land and sea 

 breezes, and has observe<l in these regions the simultaneous 

 rise of two close parallel clouds from the edge of the cumulus ; 

 such clouds are scarcely distinguishable except with oblique 

 illumination, and it is within, or between, them that the dis- 

 charges occur. The lime of their formatiim depends on the 

 hour at which the sea breeze .sets in, being roughly three hours 

 later. The land breeze l)eing dry and dusty is negatively 

 charged, while the sea breeze is known lo carry a strong positive 

 charge ; equalisation of the electrical states of the clouds 

 formed out of these will, therefore, give rise to lightning. Prof 

 Smith referred to the iridescence or nacreous appearance of the 

 edges of the clouds when rapidly sinking, and considered this 

 eflect to be due to the dust lel^t behind by them. 



This paper gave rise lo an interesting discussion, chiefly with 

 reference lo the origin of dust in clouds, and the source "of their 

 electricity. .Mr. John Aitken pointed out that thunderstorms 

 are most probably the effect, not the cause, of purifying the air. 

 He gave instances of thunderstorms on several successive days, 

 all of which left the air dusly aiul impure ; eventually the air 

 cleared, and no more thunder occurred. Prof. .Schuster alludeil 

 to the fact that twenty-five theories of ihumlorstorms had been 

 put forward in a dozen years, and in a single year five appeared. 

 He attribuled Ihe positive charge of the sea breeze to the elec- 

 trification of the air l)y the spray from the breaking waves ; 

 Lenard has .shown that the spray of pure water gives a negative 

 charge to the air, while that of salt water comnumicates a 

 positive charge. He believed Ihe dust of clouds to be .acquired 

 locally, except that at high altitudes, which we know to lie 

 carried long distances. \ proof of this is lo be found in tlie 

 liimal.ayas where certain valleys are dusty and others fairly free 

 from dust, although all receive the wind from the Indian plains. 

 His observations of nacreous clouds in Knglanil had led him to 

 connect them rather with the ice ]iarticles of ciirus clouds than 

 with dust. To this latter point Prof Michie Smith replied that 

 the nacreous appearance fits the edge of the cumulus so closely 

 that he believes the two lo be C(Uinected. 



The Committee on the application of photography to 

 meteorology are proceeding with the photograi)hy of clouds near 

 the sun by means of two cameras at a fixed distance apart, and 

 exposed simultaneously by an electrical arrangement. In this 

 way they hope lo obtain absolute measurements of cloud alti- 

 tudes. Kor (nnposes of measurement the .sun's image appears in 

 all Ihe photographs. .\ photograph of the rainljow, liy Mr. 

 .\ndrews of Coventry, is Ihe first of its kind received by ihe 

 Committee. It shows the secondary bow, and the greater 

 brightness of the regiim within the bow. 



During a recent visit to the ICngadine, Prof. Schuster has 

 made observations on the atmospheric electricity near the ground 

 at diflcreni heights above sea-level. The readings were 

 taken with Lord Kelvin's portable electromeler, which worked 

 very .satisfactorily and seems well adapted for such purposes. In 

 all cases positive charges were found, increasing with heiglil but 

 in an api>aiently erratic fashion. The normal positive charge al 

 the foot of a glacier was found to be strengthened by a wind 

 blowing down il, and Lenard's observations on the negative 



