1^ 



NATURE 



\ynne 20, 1872 



cnrlh is sufficient to cause the point to dip into the cup of 

 mercury. This completes a galvanic circuit, which stops 

 a clock at the exact half second at which the shock 

 occurred, and rin^s a bell to call the observer, and also 

 docs other woik which we shall speak of again. There are 

 three or four helices of wire of different strengths, whicli 

 support small magnets above a cup of iron filings. When 

 a vertical shock occurs some of these magnets dip into 

 the iron tilings. To one of these a light index is attached 

 for measuring the intensity of the shock. 



For horizontal shocks there are four glass tubes. Each 

 of them is bent twice at right angles, so as to form a U 

 tube. One arm of this tube has more than double the 

 diameter of the other, and is shorter. The four tubes 

 point inthe directions of the four cardinal points. Each tube 

 has a certain quantity of mercury poured into it, and on 

 the surface of the mercury, within the narrow arm of the 

 tube, there rests a small weight attached to a silk fibre, 

 which passes over a delicate ivory pulley, and has a coun- 

 terpoise attached at the other end. Each pulley has an 

 index and circular scale to mark the angle turned through. 

 The extremity of a wire is fixed at a small distance above 

 the surface of the mercury in each tube. If then a hori- 

 zontal shock occur, the mercury rises in the corresponding 

 tube ; but it rises higher in that one which has its long arm 

 to the north. The pulley is turned through a certain angle, 

 which is measured by the index, and at the same time the 

 mercury in rising comes in contact with the fixed wire, 

 and so completes a galvanic circuit which rings a bell, 

 and stops the clock at the exact half second when the 

 shock occurred. If the shock comes from some inter- 

 mediate point two of the indices will be moved, and 

 the direction and intensity can be measured by ob- 

 serving both of them. We have seen up to this point 

 that the instrument will measure the direction and in- 

 tensity of a shock, will mark the time at which the 

 shock occurred, and will ring a bell to attract the atten- 

 tion of the observer on duty, who may register succeeding 

 shocks, or, if the earthquake has ceased, may reset the 

 apparatus. But this is not all. The galvanic circuit, 

 which is completed at the moment a shock occurs, re- 

 leases at the same instant the pendulum of a second 

 clock, which has been held out of the vertical by means 

 of a detent. This clock allows a roll of paper to be un- 

 wound off a dnmi, as in any registering telegraph, at the 

 rate of three metres an hour. A pencil rests nearly in 

 contact with the strip of paper. It is connected with one 

 arm of a lever, the other arm of which is slightly distant 

 from an electro-magnet. As often as the current passes 

 this end of the lever is attracted to the magnet, and the 

 pencil in consequence is made to press on the paper, to 

 be released only when the current ceases. By this means 

 then a continuous history of the earth's trembling is regis- 

 tered, a pencil mark corresponding to a time of trembling, 

 and a blank space to a period of cessation. 



This instrument is extremely delicate, and registers 

 motions of the earth which are too slight to be perceptible 

 to the human frame. When we examined it some one 

 happened accidentally to touch the casing of the instru- 

 ment. The alarm was immediately given by the bell, and 

 the two clocks were respectively checked and put in 

 motion by the galvanic current. 



The accompanying figure (borrowed from the En- 

 gineer, for the use of which we are indebted to the courtesy 

 of the editor of that journal) may help to make 

 the above description more intelligible. In Fig. r, 

 the clock A is shown with the pendulum arrested, 

 as after a shock has occurred. The pendulum of the 

 clock B is in a position ready to be set free when a 

 shock occurs. At the same time the strip of paper I: k k 

 ■will be rolled on to the cylinder z", and at each trembling 

 of the earth the electro-magnet D will cause a pencil to 

 make a mark on the paper at the point in. P and R are 

 two pillars, between which are shown the U tubes con- 



taining the mercury, the pulleys and indices are shown 

 above. These pillars and tubes are also shown in plan. 

 Metallic bars are seen connected with R, and passing over 

 the short arms of the tubes. From these hang the wires 

 that dip into the mercury. From the pillar P, metallic 

 bars are also shown passing over the long arms of the 

 tubes ; to these are attached the wires that are almost in 

 contact with the mercury, and which complete the circuit 

 when a shock occurs. The metallic spring K, supported 

 by the pillar T, above the cup of mercury f, is the 

 apparatus for making a current during a vertical shock ; 

 Ii h h are the springs with magnets attached, which dip 

 into iron filings. The index for vertical shocks is shown 

 more clearly in Fig. 2. 



For more violent shocks the heavy bob of a freely sus- 

 pended pendulum is placed in the centre of a horizontal 

 ring in which eight tubes are placed lightly, all pointing 

 to the centre. From whatever direction a horizontal 

 shock comes it will drive out one of these tubes. The 

 tube driven out will show the direction of the shock, and 

 the distance to which it is driven will show the intensity. 

 This is also shown in plan (Fig. 3). The hole for the 

 tube is also represented. There is also shown in section 

 a cup of mercury, placed at the foot of the pillar G 

 (Fig. i), which has eight holes in its circumference just 

 above the surface of the mercury. When a shock 

 occurs mercury is driven out into the hole corresponding 

 to the direction of the shock. The quantity of mercury 

 determines the intensity. The battery is shown in Fig. 4, 

 and needs no explanation. 



In the same room there is apparatus for detecting and 

 measuring atmospheric electricity. A gold leaf electro- 

 scope and a bifilar electrometer are observed regularly. 

 These are successively put in connection with the con- 

 ductor. This consists of a disc of metal above the roof 

 of the house connected with an insulated metallic rod, 

 supported vertically, and capable of being rapidly raised 

 by means of a cord passing over a pulley. When not in 

 use this rod is in connection with the ground. In making 

 an observation the rod with the disc attached is quickly 

 raised, thereby disconnecting it from the ground. The 

 electricity of the atmosphere at the point where the disc 

 is fixed affects the electroscope and electrometer. Prof. 

 I'almieri prefers the conductor above-described to a con- 

 ducting point or a flame, because he considers that these 

 do not give comparable results, an objection which is not 

 supported by all observers. He considers the same to be 

 true of the method of dropping water. 



After having made carelul observations on atmospheric 

 electricity for about a quarter of a century in a country 

 where meteorological changes are more regular and less 

 capricious than in our own island, there is no one whose 

 deductions are more deserving of our attention ; the more 

 so as he considers that he has combined his researches 

 into a definite law. His first fact is this .—J/ within a 

 distance of about fifty miles tlierc is no shoioer of rain, 

 liail, or snoiv, tlic electricity is always positive. Tlie single 

 exception is during the projection of ashes from the crater 

 of Vesuvius. During a shower he finds the following 

 law universally to hola good : — At the place of the shower 

 there is a strong development of positive electricity ; 

 round this there is a zone of negative, and beyond this 

 again positive. The nature of the electricity observed 

 depends upon the position of the observer with respect to 

 the shower, and the phenomena will change according to 

 the direction in which the shower is moving. Sometimes 

 negative electricity may be observed during a shower ; 

 but this is always due to a more powerful shower farther 

 off. These conclusions have been supported by means of 

 telegraphic communication with neighbouring districts. 

 It appears, then, that except when the moisture of the air 

 is being condensed, there is no unusual development of 

 electricity. These results are in accordance with the ex- 

 periments of Palmieri and others, which show that 



