March 2, 1896. 



KNOWLEDGE. 



oo 



wave-crests, combined with the height of the waves, 

 enables him to calculate the velocity of the swinging 

 water particles on which depends the force he has to 

 deal with. There has been much discussion as to the 

 proper section which should be given to a breakwater 

 in order that it may best resist the force of the waves. 

 In deep water a vertical wall is pi-esumably the most 

 efficient. A vertical wall changes the running waves 

 in its vicinity into atatioiinri/ irares, in which the water 

 simply oscillates up and down without any forward 

 swhig. This results from the reflection of the wave from 

 the wall, the coincidence of a wave travelling towards the 

 wall and a reflected wave travelling seawards giving rise to 

 a wave which does not travel at all, but which retains the 

 vertical motion, so that the position which is at one 

 moment a crest is at the next a trough, and so on. 

 Something of the same kind may be noticed where the sea 

 rises and falls aroimd a sunken rock. The stationary water- 

 waves produced by reflection are very different from the 

 standing waves in shallow running streams, where each 

 crest and each trough maintains an independent fixed 

 position. A vertical breakwater, although it has great 

 advantages, is difficult and costly to construct ; and where 

 material is abundant and harbour space not too limited, a 

 common method is to " dump " down a great quantity of 

 rock, leaving the waves to lick the material into shape. 

 This is how the Portland breakwater was made, and Fig. 2 

 shows the form of section to which it has lieen brought 

 by the action of the waves. On the seaward side there is 

 first a long, gently sloping beach where the breakers act, and 

 below this is a shorter, steeper slope, where the material lies 

 very nearly at the angle of repose which it would assume 

 under the action of gravity. The form of the breakwater 

 is said to be practically permanent, in spite of the fact 

 that it is constructed of loose materials, It must be borne in 

 mind that the breakwater is not situated like a sea-wall, 

 but is supported by the water on the harbour side. 



When waves are raised by wind we are not practically 

 concerned with the expenditure of energy in their forma- 

 tion : but it is otherwise with the waves raised by ships, 

 which retard the progress of the vessel by using up a part 

 of her energj' of motion. The resistance which water 

 oflFers to the progress of a ship is of two kinds : skm 

 resistance and wave-making resistance. As the speed of 

 a ship increases, the wave-making resistance increases 

 much more rapidly than the skin resistance, so that at 

 high speeds the wave-making resistance may become equal 

 to, if it do not exceed, the skin resistance ; in other words, 

 at high speeds one half of the resistance to a ship's motion 

 may be due to the work of wave-making. Where, as in 

 the case of torpedo-boats, high speed has to be attained 

 by a vessel of small size, it is especially important that 

 all the circumstances of wave-making should be carefully 

 considered in settling the lines of the vessel. 



In our next article we shall describe in detail the beau- 

 tiful patteru of waves which is formed in the track of a ship. 



OUR FUR PRODUCERS.-II. 



SABLE, MINK, ERMINE, AND RACCOONS. 

 By E. Lydekker, B.A.Cantab., F.R.S. 



IN the preceding article of the present series we dis- 

 cussed the more aberrant fur-yielding members of 

 the .^fiixti'liilfr, and there now remain the typical 

 representatives of that family, or those which may 

 be included in the genus Muxtela. All these animals, 

 which are severally known as martens, ermines, polecats, 



and weasels, are characterized by their long bodies and short 

 hmbs ; and nearly all \'ield fur of a certain commercial 

 value, while that of some species commands a very high 

 price in the market. 



To the larger members of the group, or those charac- 

 terized by the possession of five pairs of upper and six of 

 lower cheek-teeth, the general name of martens might be 

 applied. Foremost among these, on account of the high 

 value of its fur, stands the sable, or Siberian marten (3/. 

 ^ilielUna), of Eastern Siberia and Kamschatka, which 

 is a near relative of our own yellow-breasted or pine- 

 marten (.V. iiiiirtex). Indeed, there is very considerable 

 doubt whether the sable is entitled to rank as anything 

 more than a local race of the latter species, distinguished 

 by its finer and more abundant firr. In the pine-marteii 

 the general colour of the pellage is rich dark brown, but 

 the breast has a light patch of variable size, which is 

 I generally some shade of yellow, although its tint may be 

 anything between orange and creamy white ; the under- 

 fur being reddish grey with yellow tips. The usual length 

 of the head and body varies from about sixteen to eighteen 

 inches, while the bushy tail measures from nine inches to 

 a foot. In the sable there is less variability of colour, 

 brown and dark brown bting the general hue of the fur ; 

 but occasionally light brown exam;)les, or specimens with 

 an admixture of silvery hairs, are met with, and in rare 

 instances perfectly white sables are found. Light-coloured 

 varieties appear to be most common in Kamschatka, which 

 yields the largest quantity of sable that comes to market ; 

 whereas Siberian skins are generally the darkest, and 

 consequently the most valuable. The highest priced pelts 

 come from the province of Yakutsk ; and next to these are 

 those from Okhotsk, although they are of smaller size and 

 less deep colour. The length and quality of the fur varies 

 much with the season, sable killed in summer being of but 

 slight value, and those killed in winter alone commanding 

 a high price. Sable are always skinned entire from the 

 tail forwards, so that the pelt, when removed, forms a bag. 



For their size, winter sable-skins command, perhaps, the 

 highest price of any pelts. In Kamschatka, according 

 to Dr. Guillemard, the peasants receive, on an average, 

 about sixteen roubles* per skin ; but in St. Petersburg 

 the price of the pelts varies from two pounds to upwards 

 of twenty-five pounds each, and it is stated that as much 

 as thirty-three pounds has been given for unusually fine 

 dark-coloured specimens from Yakutsk. Silvery pelts are 

 those for which there is the greatest demand iu Russia, 

 while dark specimens meet with the greatest favour in the 

 markets of London and Paris. From Amurland, where 

 Mr. Poland states from twelve thousand to twenty 

 thousand sables are killed yearly, the greater number 

 of skins are exported to China, where they are used for 

 the robes of the Mandarins. In these the tails are not 

 required, so the latter find their way into the London 

 market, where they sell at from two to six shillings apiece, 

 and are made into boas and trimmings. In Kamschatka 

 about two thousand sables are annually caught ; and in 

 1891 the number of skins sold in London alone exceeded 

 nine thousand. 



In Kamschatka, Dr. Guillemard writes that " there are 

 various methods employed in catching sables, but a less 

 number are trapped now than used formerly to be the case. 

 Dogs are almost invariably employed to run them down 

 in the deep snow or to tree them ; and they are also smelt 

 out by these trained animals in their holes at the roots of 

 trees. The great object is to tree the sable, if possible. 



• Tlu' value of t1u> )iinn'r iviilili' (>< alwiit luilf a crown, ami that of 

 the silver roulilc three shillings ami sixpeiire. 



