59^ 



NATURE 



\QcL 3, 1878 



of the prongs of the fork on the air ? A simple experiment 

 will answer this question. 



Place three lighted candles on the table at A, B, and C 

 (Fig. 13). Hold the hands upright, with the space 

 between the palms opposite A, while the backs of the 

 hands face the candles B and C. Now move the hands 

 near each other, then separate them, and make these 

 motions steadily and not too quickly. You thus repeat 

 the motions of the prongs of the fork. While vibrating 

 the hands observe attentively the flames of the candles. 

 When the hands are coming nearer each other, the air is 



Fig. 13. 



forced out from between them, and a puff of air is driven 

 against the flame A, as is shown by its bending away from 

 the hands. But, during the above movement, the backs 

 of the hands have drawn the flame toward them, as shown 

 in Fig. 13. When the hands are separating, the air 

 rushes in between them, and the flame A is drawn toward 

 the hands by this motion of the air, while at the same 

 time the flames at b and c are driven away from the 

 backs of the hands. From this experiment it is seen that 

 the space between the prongs and the faces of the prongs 



|E". ' Fig- i4- iMi' : 



of a f 6rk are, at the same instant, always acting oppositely 

 on the air. 



This will be made clearer by the study of the diagram, 

 Fig. 14. 



This figure supposes the student looking down on the 

 tops of the prongs of the fork. Imagine the pron^ 

 swinging away from each other in their vibration. Then 

 the action of the faces c and e on the air is to condense it, 

 and this condensation tends to spread all around the 

 lork. But, by the same movement, the space rr between 



the prongs is enlarged, and hence a rarefaction is made 

 there. This rarefaction also spreads all around the fork. 

 But, as the condensations produced at c and c and the 

 rarefaction at r and r spread with the same velocity, it 

 follows that they must meet along the dotted lines q, q, q, q, 

 drawn from the edges of the fork outwards. The black 

 :j-circle lines around the fork in Fig. 14 represent the 

 middle of the condensed shells of air, while the dotted 

 ^-circle lines stand for the middle of the rarefied shells 

 of air. 



Now what must happen along these dotted lines, or, 

 rather, surfaces? Evidently there is a struggle here 

 between the condensations and the rarefactions. The 

 former tend to make the molecules of air go nearer to- 

 gether, the latter try to separate them ; but, as these 

 actions are equal, and as the air is pulled in opposite 

 directions at the same time, it remains at rest — does not 

 vibrate. Therefore, along the surfaces q, q, q, q, there is 

 silence. When the prongs vibrate toward each other they 

 make the reverse actions on the air ; that is, rarefactions 

 are now sent out from c and c, while condensations are 

 sent from r and r, but^ the same effect of silence along 

 9j ?? S'j 9 is produced. 



Experiment 15. — That this is so is readily proved by 

 the following simple experiment : — Vibrate the fork and 

 hold it upright near the ear. Now slowly turn it round. 

 During one revolution of the fork on its foot you will per- 

 ceive that the sound goes through four changes. Four 

 times it was loud, and four times it was almost, if not 

 quite gone. Twirl the fork before the ear of a com- 

 panion ; he will tell you when it makes the loudest 

 sound and when it becomes silent. You will find that 

 when it is loudest the faces c, c of the prongs, or the 

 spaces r, r between them, are facing his ear ; and when 

 he tells you that there is 'silence you will find that the 

 edges of the fork, that is, the planes q, q, q, q, are toward 

 his ear. 



{To be continued!) 



ON AN ASCENT OF MO UNI HEKLA, AND ON 

 THE ERUPTION OF FEBRUARY 27, 1878 



ON February 27 last severe earthquakes were felt 

 throughout the south-west portion of Iceland, par- 

 ticularly in the districts of Land, Rangarollir, Hreppa> 

 and Fljotschlith, which are situated immediately to the 

 south and south-west of Mount Hekla. Between 8 and 

 9 P.M. an intense illumination of the sky, at first believed 

 to be actual fire, was seen to the south-east. This was 

 found to be due to the reflection by clouds of the light 

 emitted by molten lava within a subsidiary crater, or 

 bocca del fuoco, as the Italians would call it, of Hekla. 

 On the following day dense columns of smoke ascended 

 from the crater, and quantities of volcanic ashes fell in 

 the districts of Hreppa and Biskupstundur. The light 

 was seen at Reykjavik, nearly seventy miles distant, and 

 there appeared to be two vents of fire. 



One month after the eruption Prof. Tdmas Hallgrim- 

 son visited the district and endeavoured to discover the 

 exact position of the new crater. He found it in the 

 Raudaskal Valley, about four miles to the north-east of 

 Hekla, and in connection with one of its outlying spurs. 

 The chief crater was observed to be near the northern 

 base of Krakatinkr, and a good deal of new lava was 

 heaped around it. Herr Nielsens, a merchant of Eyra- 

 bakki, on the southern coast of Iceland, visited the scene 

 of the eruption about the same time, and by ascending 

 Krakatinkr he Avas able to look down into the new crater. 

 He also determined its position, and traced the course of 

 the new lava streams. The map which is here reproduced 

 (for which I am indebted to Sjera Gudmundr Jonsson,, 

 the priest of Stdruvellir, a hamlet near to Hekla) is a 

 copy of Nielsens' sketch made on the spot. 



Upwards of a month ago (August 21) I visited the scene 



