5§4 



NATURE 



{Oct. 1 6, 1884 



Circular Rainbow 



In the notice given in Nature (Sept. 11, p. 465) of the beauti- 

 ful circular rainbow that is seen in the spray of the Montmorenci 

 Falls near Quebec, the expression in the heading, " seen from a 

 hill-top " will convey an erroneous idea without some explana- 

 tion. The complete circle is only seen by getting down in the 

 spray to the edge of the low rocks, within a few inches of the 

 level of the water, and the circular bow then passes down to the 

 feet on each side ; it is indeed most perfectly seen by turning 

 round and stooping down to look back between the legs, when 

 tlie complete circle is seen without interruption from the feet. 

 The bow is small in diameter, and is a narrow band, appearing 

 nearer to the eye than an ordinary spray rainbow. I had the 

 pleasure of seeing it on August 25, on the occasion of the British 

 Association Canadian visit. William P. Marshall 



15, Augustus Road, Birmingham, October 13 



To Find the Cube of any Number by Construction 

 Prof. Karl Pearson has kindly referred me for a simple 

 graphical construction for any positive or negative power of any 

 rational quantity whatever to Egger's " Grundziige einer graph- 

 ischen Arithmetik." This method, he informs me, is repro- 

 duced in Cremona's "II calcolo graphico." I was of course 

 aware that there were several simple constructions. I was in- 

 duced to write upon the subject owing to the unexpected dis- 

 covery that there was a line in the geometry of the triangle 

 which enabled one to obtain the cube of a number. 



October 14 R. Tucker 



EXP LOR A TIONS IN ICELAND 1 

 The Lava Desert of OdASahraun 



ON July 25 we set out for the southern Dyngjufjoll, in 

 order to examine Askja. All previous explorers of 

 that volcanic locality have taken the northern route from 

 Svartarkot, but no one has hitherto approached it from 

 the east, from HerSubreiS, any advance from that side 

 having been deemed impracticable. This I wanted to 

 test for myself, and shaped my course from the tent 

 (pitched, as before said, to the south of HerSubreiS) in a 

 direct line on the wide gap that opens in Askja to the 

 east. The whole intervening country was one continuous 

 succession of lavas, so effectively covered with pumice 

 and scoria; from the great explosion of 1S75, fortunately 

 for us, that the whole was really one scoriae plain, the 

 pumice boulders measuring generally one to two cubic 

 feet, some more, some less. If it had not been for this 

 scoriaceous cover, these lavas would have proved pretty- 

 certainly utterly impassable for horses. We took good care 

 to keep to the crests of the thickest pumice-drifts, and 

 though such travelling is rough enough for horses, yet 

 they sustain no great harm, because the pumice is so 

 light and brittle. Under the south-eastern spurs of 

 Dyngjufjoll we came upon a lake, shallow, but of con- 

 siderable magnitude, of the existence of which there was 

 no previous knowledge. About midway between HerSu- 

 breiS and Dyngjufjoll the country begins to rise up 

 towards the aforementioned gap in Askja. Askja is a 

 cauldron-shaped valley in the centre of Dyngjufjoll, which 

 is an enormous complex of mountains 4500 feet high. 

 This valley contains innumerable craters which have 

 erupted at various periods ; the sides of this valley rise to 

 between 700 and 800 feet, but out of the aforementioned 

 gap lavas have flowed over the lower country outside all 

 the way down to Odafcahraun, forming an enormous oval 

 of an average incline of 4 33'. When we came close 

 up to the gap, the scoriae ceased, and at once the lava 

 became exceedingly difficult to pass. But by aid of frozen 

 snowdrifts filling dips and dints in the slopes, we managed 

 to thread our way along, and thus actually to get into the 

 valley ; only one single tongue of lava we had to cross 

 without the aid of snowdrifts — one which, though very 

 narrow, we had the greatest difficulty in getting our ponies 



1 Continued from p. 565. 



over. Having at last succeeded in this, we rode along 

 frozen snowdrifts under the southern slopes of the Askja 

 valley, and thus reached actually on horseback the craters 

 which exploded here in 1S75. Previous visitors to Askja 

 have entered the valley through a pass in the mountains 

 inclosing the valley from the north, outside which pass 

 they have had to abandon their horses and to reach the 

 craters on foot over an almost impassable lava-stretch in 

 the bottom of the valley, taking four to five hours in 

 passing the distance from the pass to the craters. 

 From our tent by Lindaa it took us nine hours to 

 reach the craters, but the return route we accom- 

 plished in seven. We now left our ponies provided 

 with their fodder beside the latge emptor of 1875, ar >d 

 set off on foot to examine the locality in every direction, 

 spending for that purpose the whole of the bright night 

 and a portion of the next day. So over-covered was 

 Askja with snow that journeying along here was like 

 journeying in the heart of winter. The whole mass of 

 Dyngjufjoll is made up of palagonite breccia inter- 

 spersed with layers of basalt. Into this mass Askja 

 sinks in the shape of a shallow basin, and may derive 

 its present form partly from certain stretches of it 

 having sunk down in consequence of eruptions, partly 

 from that natural dint or basin-formation of valleys 

 which is so strikingly common to tufa mountains in Ice- 

 land. But the supposition that the whole of this valley, 

 about sixteen square miles English, is one crater, the 

 result of one great volcanic explosion, is unwarranted. In 

 the great eruption of 1875 a very considerable extent of 

 ground " fell in " in the south-eastern corner of the Askja 

 valley round the craters, and the vertical precipice of the 

 fractured crust of the earth on the side of the Askja valley 

 measures, according to Prof. Johnstrup's survey, 740 

 Danish feet ; that at the opposite side in the mountains is 

 at least double in thickness. The vertical walls of the 

 precipices exhibit in a clear manner the successive layers 

 of lava which fill the bottom of the Askja valley. In the 

 earth-slip thus created there was, in 1876, a small lake of 

 dull-green colour, circular, and measuring about 4000 feet 

 in diameter. This lake now fills the whole bottom of the 

 slip and measures 10,000 feet in length. In 1876 the 

 temperature of the water was 22 Celsius (7i°'6 F.), but 

 has now fallen to 14 C. (57°'2 F.). The crater, which 

 by its explosion covered the east country with pumice 

 and scoriae in 1875, is situated in the north-eastern 

 brim of the fissure, and is 300 feet in diameter and 150 

 feet deep ; its outer circumference flat, and built up of 

 scoriae ashes, its inside cylindric and perpendicular. In 

 1876 this crater only emitted steam, now it has turned 

 into a boiling cauldron of clay, the clay mud at the 

 bottom being gray with an admixture of bluish green tint, 

 boiling and wallopping incessantly ; through the south- 

 eastern part of its bottom there issues by a subterranean 

 vent a thick column of steam with loud roars and reports, 

 and all around this column smaller fissures issue thinner 

 jets of steam and fumes. Interspersed with the scoriae in 

 Askja and on the eastern side of the surrounding moun- 

 tains are found small glazed grayish-blue pieces of 

 trachyte thus formed by the last eruption : among these 

 there are some found of which one-half, or a portion, is 

 reduced to pumice, while the remainder retains its tra- 

 chytic constituency. In the south-eastern corner of the 

 dip right up from the water are also found a number of 

 craters from which radiate rents and gulfs honeycombed 

 with innumerable fumaroles and crater-tubes from which 

 clouds of steam roll up high above the crests of the 

 mountains, the roar and boom from which are heard to a 

 great distance, resembling the rumbling sound of steam let 

 off from many boilers at once. Deposits of sulphur are 

 already visible round a number of the fumaroles, and 

 yellow-green patches of sulphur show all about the preci- 

 pices, where every chink and rupture lets off sulphurous 

 fumes. In the eastern part of the slip the scoriaceous lay ers 



