Jan. 17, 1889] 



NATURE 



27 1- 



Mr. Worth ington speaks as if we were anxious to do away 

 with a student's familiaiity with force as a push or a pull. This 

 shows that he does not appreciate our position, 



I even venture to assert that in what he says concerning nr.ass 

 and inertia he is not so absolutely clear in his own mind as it is 

 desirable for a reformer to be. May I suggest to him that the 

 "inertia " or " inenia- react ion " of a lump or mass of matter — 

 that which is measured in an experiment, and the only thing 



that can be measured in an inertia experimtnt — is vi~. ; and 



that the coefficient of the otherwise measurable kinematic factor 

 I n this qu-intity is properly called "the coefficient of inertia, ' 

 but is, for brevity, styled " mass," and is taken as a measure of 

 the quantity of matter in the body, because, experimentally, it 

 is found to be absolutely unalterable by every physical and 

 chemical process except those which change the amount of 

 matter in the lump. Fancy making our standard of quantity of 

 matter depend upon the approximately determined gravitative 

 attraction of some arbitrarily selected planet at some arbitrarily 

 selected spot near its present surface I 



Sometimes, iinleed, m is briefly called merely " inertia," just 



as the coefficient — ^ - in Ohm's law is for brevity styled 



"resistance"; but the full names of these quantities are 

 "coefficient of inertia" and "coefficient of resistance," 

 respectively. In the case of friction the full name is usually 

 given. With junior students it is clearest to give the full names 

 in every case ; just as it is much clearer with them to avoid the 

 misleading abbreviation specific heat, and to use the full phrase 

 specific capacity for heat. Oliver J. Lodge. 



Liverpool, January 14. 



A Hare at Sea. 



Among the notes published in Nature for December 27, 

 1S88, is an account of a hare swimming across a river ; perhaps 

 the following account of a hare taking to the sta may be of 

 interest. In October 1887, I was a member of a shooting party 

 staying near Auchencairn on the Kirkcudbrightshire coast, 

 where for miles the waves of the Solway beat on red sandstone 

 cliffs, broken here and there by small bays, where the burns run 

 down to the sea through little glens. One day I had left the 

 others and was standing among the seaweed- covered boulders of 

 such a bay, when the sounds of a course reached me from a hill- 

 side a quarter of a mile or more away, and presently I saw hare 

 and greyhounds coiring down to the shore ; they ran close past 

 where I was standing, and then to my astonishment the hare 

 deliberately entered the water and swam out to sea. 



I could not persuade the greyhounds to follow, though one 

 was so clo>e that, if she had done so at once, she could have 

 caught the hare without swimming, as the latter was out of her 

 depth directly and swam very slowly. The sun was shining very 

 bright on the water, and it soon became very difficult to keep the 

 hare in sight, as her head only showed now and then on the top 

 of a wave, and about a hundred yards from shore I saw her for 

 the last time, though I stayed about the place a long while. 



This hare was perhaps hard pressed, still I could see no reason 

 why she should not have run along the shore to the march 

 dyke, which was close to, and where she would probably have 

 made good her escape. W. J. Beaumont. 



Sandiway, North wich, January 13. 



THE ARTIFICIAL REPRODUCTION OF 

 VOLCANIC ROCKS} 



r^RIGINALLY, the study of the crust of the earth 

 ^^ was purely utilitarian : it seems to have been at 

 first forced upon man by the necessity of exploring the 

 strata in order to extract metallic ores, constructive 

 materials, and combustible minerals. 



To anyone who glances at the history of the sciences, 

 it becomes evident that they all owe their origin to some 

 useful and practical aim, and that from this initial phase 

 they have passed through a regular development : this 



A Lecture delivered in French at the Royal Insiiiution, tn FVid.iy 

 May 18, i888, bv W. Alphonse Renard, LL.D., Hon.M.R.S.E., Corr G S , 

 Curator of the Royal Museum, Brufsels. Translated by F. W. Rudler. I 



progress, so far as geology is concerned, I shall proceed 

 to sketch. 



Man, then, commences to explore the depths of the 

 earth in order to extract the materials which may minister 

 to his wants. At first he works without rule ; but as the 

 miner's art is developed, method is introduced into the 

 search for mineral wealth, and he observes the conditions 

 under which useful minerals and rocks occur in the bosom 

 of the earth. These observations, at first merely empirical 

 and local, gradually become generalized, and thus lead to 

 a recognition of some of the leading features in the archi- 

 tecture of our planet. On digging into the earth, it soon- 

 becomes evident that the world was not made at a single 

 stroke, but owes its formation to a succession of epochs. 



It follows, therefore, that, in order to interpret the 

 history of the earth, and the operation of the agencies 

 which have taken part in its formation, it is necessary to 

 study the living world, and to investigate the present 

 condition of our planet. In comparing the various strata 

 of the earth with the deposits which are in course of 

 formation under our own eyes, we realize the conditions 

 which have presided at the formation of the stratified 

 rocks of ancient geological periods. It is thus that, by 

 the analysis of facts, and by induction which generalizes 

 the observations, our knowledge of the crust of the earth 

 enters on a new and truly scientific phase. We start by 

 attempt-ng to discover practical rules for the guidance 

 of the miner, and we are gradually led to decipher the 

 history of the earth. 



In this reconstruction of the past history of our planet 

 we are guided by a fundamental principle — namely, that 

 the essence of the forces which have acted upon the 

 earth has never changed. We ought, then, to seek in 

 geological epochs for traces of only such phenomena as 

 are of the same nature as those which we can witness 

 to-day, and submit to direct observation. 



Since geologists commenced, towards the close of the 

 last century, to apply the inductive method to the study of 

 the mineral masses which form the crust of the earth, to 

 their architecture, and to the organic remains embedded 

 in the rocks, a vast collection of documents has accumu- 

 lated, bearing upon the history of our planet. During 

 this period, Geology has made such immense progress 

 that she need not envy the older branches of natural 

 science. 



Let us see how, in applying this analytical method and 

 relying on induction, geology interprets the formation of 

 the rocks. Rocks, we kn6w, are the solid mineral masses 

 which constitute the earth's crust. Observation teaches 

 us to recognize two groups. The first are characterized by 

 an arrangement in beds or strata : these are the sediment- 

 ary rocks. The second group, which does not present 

 this stratified arrangement, comprises rocks of volcanic 

 character, with a massive structure. These differences 

 in the structure and composition of the two great litho- 

 logical groups lead us to regard them as having been 

 formed under special conditions, which have left their 

 imprint upon each group. 



We see the sedimentary rocks in the course of forma- 

 tion when we observe how detrital matter is rolled about 

 by stream and wave, and how such waters deposit 

 pebbles, sand, and mud upon their beds. After the 

 death of the organisms which inhabit these waters, theii 

 skeletons or their shells becoine mingled with the mineral 

 deposits, and with them build up sedimentary masses. 

 The minerals so deposited assume, by successive accu- 

 mulation, a stratified arrangement. All their constituent 

 particles were originally isolated grains, and still retain 

 traces of their origin : they are either the di'bris of pre- 

 existing rocks or organic exuviae, which, by physical 

 and chemical processes, may become subsequently 

 consolidated. 



Let us now compare these modern sedimentary 

 deposits, characterized by a stratified arrangement, and 



