Feb, 7, 1878] 



NATURE 



281 



my communication to Nature, vol. xvii. p. 183, to show the 

 latter fact as far as the observations go up to the present time. 

 I did not, however, venture in that article to make a guess as to 

 the future, which really could have little weight till we have 

 another year's observations. Yet I thought it desirable, nearly 

 two months ago, to place in the hands of Prof. Balfour Stewart 

 the evidence that the possibility of such an event, as an obvious 

 conclusion from my results, had not escaped me. As Mr. 

 Jenkins has published his guess, I may do the same with mine. 

 The latter differs, however, from his in a very important way. 

 He supposes the next maximum will be in 1887, whereas I 

 suppose that the weak maximum of 1797 may be repeated near 

 18S0. In this latter case the interval between the two will be 

 nearly double that found by me (forty-two years), during which 

 the sun-spot period appears to have all its different lengths. 

 February i John Allan Brcun 



Terrestrial Magnetism 



I HEREWITH submit a notice of an experiment for illustrating 

 to a class the action of terrestrial magnetism. In a simple 

 way it clearly exhibits to a large audience the action of the 

 currents of electricity that pass around the earth. The experi- 

 ment was suggested on readina: a paper by Prof. J. W. Mallet, 

 F.R.S., of the University of Virginia, 'on " The Apparent Altera- 

 tion in Weight of a Wire placed East and West, and Traversed 

 by an Electric Current " {Phil. Mag., November, 1877). 



Instead of disconnecting the wires placed east and west from 

 the portion of the rectangle, as was done by Prof. Mallet in the 

 experiment alluded to, whereby the attractive or repulsive action 

 of the earth currents on one side only of the rectangle was 

 obtained, it occurred to me to suspend the whole rectangle to a 

 balance. Properly arranged in this way the attraction for parallel 

 currents in same direction, and repulsion for currents in opposite 

 direction would generate a couple, tending to produce rotation 

 around an east and west horizontal axis, and hence augment the 

 deflection of the balance. 



A rectangular frame was made of light poplar wood, of sec- 

 tion three by two centimetres, whose sides were one metre in 

 length by three-fourths of a metre in breadth. About the 

 perimeter of this rectangle there were wrapped twenty coils of 

 insulated copper wire. Each extremity of the wire was made to 

 terminate near the centre of one of the shorter sides, and passing 

 through the wooden frame, was fastened and cut off about 3 cm. 

 from the frame. 



This rectangular frame was then so suspended, in a horizontal 

 position, by wires attached to the pans of an ordinary Delenil's 

 hydrostatic balance, that the longer sides were at right-angles to 

 the beam. By adjusting weights In the scale pans the index of 

 the balance was brought to the zero. Two small orifices bored 

 in a block of wocd a centimetre apart, served as mercury cups, 

 in which the extremities of the short terminal wires were Im- 

 mersed ; near the bottom and through the walls of these wooden 

 mercury-cups were screened small brass hooks, which served as 

 connectors, to which the wires of the battery were attached. The 

 balance was now so placed that the longer sides of the suspended 

 rectangle were at ri^ht-angles with the magnetic meridian^ or in 

 the magnetic east and west line. 



When the current from the battery was made to pass around 

 the rectangle from east to west, 'on the northern side, and from 

 west to east, on the southern side, by the theory of terrestrial 

 magfnetlsm, the north side of the rectangle would be attracted to 

 the earth, and the south side repelled, and that this was so the 

 corresponding deflection of the balance rendered plainly visible. 

 When the current was reversed the deflection was In the opposite 

 direction. By breaking and closing the circuit at proper intervals to 

 augment the oscillations, the large frame was readily made to 

 oscillate through an arc of 5°. When the sides of the rectangle 

 were placed north-east and south-west, the current produced no 

 sensible effect. A bichromate of potash battery of sixteen cells 

 was used with plates of zinc and carbon 25 cm. by 6 cm. 



With a rectangle containing a larger number of colls of wire 

 attached to a delicate balance by the use of a constant battery, 

 the variations In the earth's magnetism might be thus observed. 



Wm. LeRoy Broun 



Vanderbilt University, Nashville, Tenn,, January n 



Seiches and Earthquakes 

 In the last number of Nature (p. 234) you make an allusion 

 to the fact that the earthquake of October 8, 1877, has not been 



traced by the self-registering " Hmnlmeters" (tide-gauge) of M. 

 Ph. Plantamour at Geneva, and myself at Merges. Let me take 

 the opportunity of the great publicity of your excellent paper to 

 ask the naturalists who live in other countries more frequently 

 visited by earthquakes, for an explanation. 



I believe I have demonstrated In many different papers that 

 the phenomenon called seiches, which consists in rhythmical 

 movements of the level of the lake, Is a balancing-wave, a wave 

 of stationary uninodal oscillation. The water moves In balanc- 

 ing oscillation In the two principal diameters of the lake. In the 

 direction of the greatest length and of the greatest breadth. 

 For setting the water In such an oscillatory movement there are 

 two possible causes : — 



1. A shock given to the water Itseli Is the most frequent 

 case, and I can prove that generally the seiches are caused 

 by some rupture of the equilibrium of the atmospheric pres- 

 sure ; many storms, and especially those that fall somewhat 

 abruptly on the lake, are accompanied by very high sexhes, 

 and I have many examples of the beginning of the oscillatory 

 movement of the water exactly at the same time the storm 

 commences. 



2. A movement of the soil on which the water lies, an earth- 

 quake. It is evident that a shock given to a basin can put the 

 water In oscillatory movement. In fact. It happens frequently. 

 I win only recall the colossal transmission- waves In the Pacific 

 Ocean on August 13, 1868, after the earthquake of Arica ; that 

 of May 9, 1877, at Iqulque ; and in earlier times, the earthquakes 

 at Messina, 1783 ; at Port-Royal (Jamaica), 1692 ; at Callao, 

 1586, &c. If such enormous waves had taken place In a closed 

 basin, as our lakes, It would probably have brought about an 

 oscillatory movement of seiches. I could bring many facts to 

 prove It ; only one example, the earthquake of Lisbon (1755), 

 was noticed In Switzerland and Germany, chiefly by the 

 movements of the water of the lakes ; the description of these 

 movements recalls perfectly the seiches. 



It is also theoretically probable that the shock given to the 

 ground extends to the waters, and that an earthquake will 

 produce seiches In a lake. Unfortunately the facts observed 

 up to this time do not confirm this theoretical view. Since I 

 established at Morges a self-registering limnlmeter of the 

 greatest sensitiveness. In March, 1876, six different earthquakes 

 have been noticed In our country, and specially three earthquakes 

 were felt at Morges Itself — May 7 and November 29, 1876, 

 and October 8, 1877. Not one of those six earthquakes has 

 been traced by the self-registering limnlmeter ; not one has Inter- 

 rupted the rhythmic oscillation of the seiches which were taking 

 place ; not the smallest alteration of the curve has shown that the 

 water had been acted upon In a peculiar manner ; neither was 

 the limnlmeter of M. Ph. Plantamour, which was at work during 

 the earthquake of October 8, 1877, Influenced by that very severe 

 shock. And yet our apparatus are extremely sensitive ; when 

 the lake Is sufficiently calm my limnlmeter can show the waves 

 originated by a steamboat which passes 10-15 kilometres 

 off the apparatus, or It registers the waves caused by a 

 steamer which has passed by my observatory two or three 

 hours before. 



How can these contradictory facts be explained? On one 

 hand, the earthquakes cause In many places enormous waves ; 

 on the other, three earthquakes strong enough to have awakened 

 men out of their sleep, have not put In movement the most 

 sensitive, always working, self-registering apparatus. 



I suppose that the shocks of the earth do not transfer always 

 the movement to the water ; that only In a special direction of 

 the shock a special Intensity, a special duration, the water Itself 

 is put in movement and takes the rhythmic oscillation of the 

 seiches. If I shake a basin the water Is not always and necessarily 

 put In oscillatory movement. I think it is the same for the 

 seiches, and I believe that only certain movements of the earth 

 cause the water of the lakes to move. 



It Is the point on which I seek an answer from naturalists 

 who have more opportunity to observe the effects of earthquakes. 

 I ask If each earthquake Is accompanied by waves of the 

 sea ; If each shock of the same Intensity Is accompanied 

 by waves of the same amplitude ; If there are not differences 

 between the different earthquakes ; if some have the enor- 

 mous waves of Iqulque or Arica ; if others are without those 

 waves ? 



I should be very thankful to receive an answer to the above 

 questions. F. A. FoREL 



Morges, Switzerland, January 24 



