March 4, 1886] 



NA TURE 



429 



ment of tliese facula; in parts of the sun where there are no 

 spots at all. 



Those who are familiar with this class of observations 

 will remember that it is much easier to see the facula; near the 

 sun's limb than in the centre of the sun. Also it is easier 

 to get a photograph of the faculce using a collodion or a dry 

 plate which works very far up in the blue, than it is with a col- 

 lodion or a dry plate which works in the green or the blue- 

 green ; this latter fact proves to us quite conclusively, as it 

 was pointed out a good many years ago now,' that the difference 

 between the light at the top of a dome, so to speak, or the 

 bottom, or between the top of the cumulus and the base of the 

 pore, is a difference chiefly of tliat kind of light which writes 

 its record by means of the absorption of the blue end of the 

 spectrum. 



The reason that we see the sun red at sunrise and sunset fre- 

 quently is not that there is anything different in our air at that 

 moment, but because we are looking at the sun through a greater 

 thickness of the air ; and the redness of the sun is the balance 

 left after our atmosphere has done all it can in the way of ab- 

 sorbing the blue. We do not expect to get the sun red at mid- 

 day. Of course a London fog will do anything ; but I am 

 talking of our ordinary atmosphere ; and the fact that we do 

 riOt get the sun red in the middle of the day is one of the same 

 kind .as the other one that we do not so easily see the faculs on 

 the centre of the sun as we do at the edge of it. There is 

 absorption going on between the top of a facula and the bottom 

 of a pore ; and, as you know, to get that out in its greatest 

 vigour and quantity we must take the greatest possible thickness 

 of atmosphere. We see in a moment that the only way to have 

 a considerable thickness of solar atmosphere to work this for 

 is to make observations near the sun's limb. 



These faculas exist on an enormous scale. It is quite common 

 to see reaches of them tens of thousands of miles long, lasting 

 for days, and perhaps weeks ; we get in that fact an indication 

 of the enormous amount of energy which may still be changing 

 places in the solar atmosphere, even though we do not get other 

 phenomena which appear to us to be more important. By 

 " other phenomena " of course I mean the spots. 



J. Norman Lockyer 

 ( To be continued. ) 



BARK BREAD 



IVyTOST travellers in Norway have probably had more than 

 •'■■'• sufficient opportunities of becoming acquainted with the 

 so-called "Fladbrod," flat bread, of the country. Few, how- 

 ever, among them who have partaken of this dry and insipid 

 food may possibly be aware that in many districts, more es- 

 pecially in Hardanger, the chief ingredient in its composition is 

 the bark of trees. This substitution of an indigestible product 

 for I'onA fide flour is not necessarily a proof of the scarcity of 

 cereals, but is to be ascribed rather to an opinion prevalent 

 among the peasant women that the bark of young pine branches, 

 or twigs of the elm, are capable of being made into a thinner 

 paste than unadulterated barley or rye-meal, of which the 

 Norse housewife, who prides herself on the lightness of her 

 "Fladbrod," puts in only enough to make the compound hold 

 together. 



The absence of any nutritive property in bark bread, whether 

 made with elm or pine bark, and the positive injury it may do 

 the digestive organs, has of late attracted much notice among 

 Norwegian physiologists, and the editor of Naturen, with a 

 view of calling the attention of the public to the subject, has, 

 with the author's permission, reprinted some remarks by 

 Dr. Schtibeler on the history and character of the bark bread of 

 Scandinavia. From this source we learn that the oldest refer- 

 ence to the use of bark bread in Norway occurs in a poem, 

 ascribed to the Skald Sighvat, who lived in the first half of the 

 eleventh century. In the year 1300 the annals of Gothland 

 record a season of dearth, in which men were forced to eat the 

 liark and leaf-buds of trees, while then, and during the later 

 periods of the Middle Ages, the frequent failure of the crops in 

 all parts of Scandinavia led to the systematic use of the bones 

 and roe of fishes, as well as the bark of trees as a substitute 

 for genuine flour ; and so extensively was the latter substance 

 used that Pastor Herman Ruge, who in 1762 wrote a treatise 

 on the preservation of woods, has drawn attention to the almost 

 ' In 1872 ; see " Solar Phyixs," p. 404. 



complete disappearance of the elm in the Bohus district, which 

 he ascribes to the universal practice in bygone times of stripping 

 the bark for the preparation of bread. 



In Nordland and Finmark the root of Stridhiopteris gertnanica 

 and other ferns, as well as the leaves of various species of 

 Rumex, have been largely used with barley-meal in making 

 ordinary bread as well as " Fladbrod." In Finland the national 

 " pettuleipa " (bark bread), which was in former times almost the 

 only breadstuft'of the country, still ranks as an ordinary article 

 of food in Kajana, and in the forest-regions of Oesterbotten. and 

 Tavastland. Here it is usually made of the inner layers of the 

 pine-bark, ground to a meal, which is mixed with a small 

 quantity of rye-flour to give the requisite tenacity to the dough. 

 The Finlanders of an older generation showed marvellous in- 

 genuity in composing breadstuffs, in which scarcely a trace of 

 any cereal could be detected in the mixture of bark, berries, 

 seeds, bulbs, and roots of wild plants, which they seem to have 

 accepted as a perfectly legitimate substitute for corn-bread. In 

 the interior of Sweden, according to Prof. Save, the best bread 

 of the peasants consisted till the middle of this century of pease, 

 oats, and barley-meal in equal proportions, while in the ordinary 

 daily bread the husks, chaft", and spikes of the oats were all 

 ground down together. In bad seasons even this was unattain- 

 able by the Dalekarlian labourer, who had to content himself 

 with pine-bark bread. 



DILA TANCY ' 

 '1"'HE princip.al object of this lecture was to show eicperimental 

 •*• evidence of a hitherto unrecognised fact of fundamental 

 importance in mechanical philosophy. This newly-recognised 

 property peculiar to granular masses (named by the author 

 "Dilatancy") would be rendered clear by the exp-riments. 

 But it was not from these experiments that it had been dis- 

 covered. This discovery was the result of an endeavour to 

 conceive the mechanical properties a medium must possess in 

 order to act the part of the all-pervading ether — transmitting 

 waves such as light, but not such as sound, allowing free motion 

 of bodies, causing distant bodies to gravitate, and causing forces 

 like cohesion, elasticity, and friction between adjacent molecules, 

 together with electricity and magnetism. 



As the result of this endeavour, it appeared that the simplest 

 conceivable medium, a mass of rigid granules in contact with 

 each other, would answer not only one but all of these require- 

 ments, provided such shape or fit could be given to the grains 

 that, while these rigidly preserved their shape, the medium 

 should possess the apparently paradoxical or anti-sponge-like 

 property of swelling in bulk when its shape was altered. 



This required that the grains should so interlock that, when 

 any change in the shape of the mass occurred, the interstices 

 between the grains should increase. Having recognised this 

 property as a necessity of the ether, the next question became, 

 What must be the shape and fit of the grains so that the mass 

 might possess this unique property ? At first it seemed that 

 there must be something special and intricate in this structure. 

 It would obviously be possessed by grains shaped to fit into 

 each other's interstices : this was illustrated by a model of bricks 

 arranged to bond as in a wall ; when the pile was distorted, 

 interstices appeared. Subsequent consideration revealed this 

 striking fact — that any shape of grains resulted in a medium 

 possessing this property of dilatancy so long as the medium 

 was continuous, or so long as precautions were taken to prevent 

 rearrangement of the grains, commencing at the outside. All 

 that was wanted was a mass of smooth hard grains, ea li grain 

 being held by the adjacent grains, and the grains on the outside 

 being so controlled as to prevent rearrangement. This was illus- 

 trated by a model of a pile of shot, which, when in closest order, 

 could not have its shape changed without opening the order and 

 mcreasing the interstices. The pile being brought from do est 

 to most open order by simply distorting its shape, the ontside 

 balls being forced, those in the interior were constrained to 

 follow, showing that in no case could a rearrangement start in 

 the interior. 



Considering the generality of this conclusion, it was neces- 

 sary to explain how it was that dilatancy was not a property of 

 ordinary atomic or molecular matter. This was owing to the 

 elasticity, cohesion, and friction which rendered molecules in- 



' Abstract of a Lecture delivered at the Royal Institution of Great 

 Britain, on Friday evening, February 12, 1886. By Prof. Osborne Reynolds, 

 LL.D.,F.R.S. 



