578 



NA TURE 



[Oct. 20, 1887 



steam-pressure at once, because the steam must first work 

 in, and be exhausted from, the high-pressure cyHnder 

 before reachhig the low-pressure cylinder. Thus, it is 

 evident that the high-pressure piston has to move the 

 engine through at least half a revolution of the driving- 

 wheels before the low-pressure piston is able to do any 

 work ; and further, if the high-pressure crank happens to 

 be at or near the dead centre, it is impossible for the engine 

 to move one way or the other, unless some means are 

 adopted to make the low-pressure piston effective to move 

 the high-pressure piston past the dead point. Mr. Wors- 

 dell's starting-gear entirely gets over this difficulty, and 

 makes the two-cylinder compound locomotive a handy 

 engine and a good starter. 



By means of this arrangement the engine-driver is able 

 to close the pipe connecting the high- and low-pressure 

 valve-chests by a valve opening towards the low-pressure 

 valve-chest. At the same time, steam direct from, the 

 boiler is admitted into the low-pressure cylinder, the 

 intercepting valve preventing it blowing back into the 

 high-pressure cylinder. On the engine moving for half 

 a revolution of the driving-wheels, the high-pressure 

 cylinder exhausts into the low-pressure valve-chest, blow- 

 ing the intercepting valve open on its way, and compound 

 working commences. As soon as the engine moves, the 

 supply of steam from the boiler is of course discontinued 

 in the low-pressure cylinder. In actual practice this gear 

 is more or less self-acting, and is set in motion by 

 moving one small lever. 



At the end of the article a useful index is given, and 

 throughout copious references are made to many authori- 

 ties, which will be of service to anyone searching for 

 information in any particular branch. 



Prof. Ewing also contributes an interesting article on 

 the strength of materials, and has treated the subject in 

 a very satisfactory manner. After a clear introduction to 

 the subject, we find a really practical description of tensile 

 testing of materials. This is very well done, including as 

 it does all recent information and experiments. After 

 the general adoption of steel in the construction of engin- 

 eering structures, whether bridges or ships, tensile testing 

 of the material came into every-day use, and a testing 

 machine is now to be found in every steel- works worthy 

 of the name. Engineers were slow in adopting steel, 

 and rightly so, considering the many unaccountable 

 failures of that material which took place a year or two 

 ago. Even now, when its manufacture is much better 

 understood, stringent tests are specified by our leading 

 engineers before the material is allowed to be used ; and, 

 as an example, all steel plates used in the bridge work for 

 certain railways are tested as follows. Every plate rolled 

 has a side and end shearing tested by tensile tests, 

 besides the usual quenched and cold bends, in order to 

 guard against the possible use of brittle or dangerous 

 steel. It is not to be wondered at, therefore, that the 

 tensile testing machine has reached a high state of per- 

 fection, the latest improvement being the addition of an 

 autographic recorder by which the results of each test 

 piece broken are recorded by the machine itself. This 

 article contains all the information likely to be required, 

 and has been put together in a concise form. Prof. Ewing 

 making the most of the limited space placed at his disposal. 

 Both articles are quite models of encyclopaedic articles. 



The principal articles relating to natural history are 

 the following: — On the "Siluridae," by Dr. Giinther. 

 Some of the more remarkable points in the structure and 

 life-history of the members of this group hiving been 

 already noticed under the article " Ichthyology," in the 

 present article we find only a notice of the chief sections 

 into which the Siluroids have been divided ; but though 

 thus short, it gives a most useful ;-/j-«w/of whatis known 

 on the subject. A long and important article on "Snakes" 

 is also from the pen of the same author. The article 

 commences with an account of the classification and 

 distribution of the members of this order, the number 

 of known species being reckoned at from 1500 to 1800; 

 next follow some details as to their general anatomy, 

 especially as to their poison-fangs. A list of the sub- 

 orders and families is given, and there are some excellent 

 illustrations of some of the more important species. The 

 various notices on birds, the snipe, stork, swan, &c , have 

 been contributed by Prof. Alfred Newton, and, it is need- 

 less to say, embody all the most recent information pos- 

 sible in the small space allowed. The article on " Swine " 

 is by Prof. Flower. A most excellent article on the 

 " Skeleton " is by Mr. St. George Mivart. While we have 

 nothing but praise for his treatment of the subject of the 

 general skeletal conditions of the Vertebrata, we may 

 venture to doubt whether the author could justify the 

 assertion which occurs in the second paragraph of the 

 article, to the effect that among plants the Desmidiae 

 clothe themselves with a horny coat. Making all 

 due allowance for the rather loose way in which this 

 word "horny" is used by some biologists, we think the 

 botanists have not yet agreed to use it for any of 

 the varieties of cellulose, and the Desmids are not 

 even very exceptional in their clothing. The most 

 important article from a zoological point of view in 

 the volume is that on " Sponges," by Prof. Sollas. 

 He takes notice of the great advance that has been ' 

 made in our knowledge of this group during the last 

 fifteen years, and gives one of the best accounts of the 

 group in our language. Beginning with a sketch of the 

 general structure and form of Sponges, he plunges into a I, 

 disquisition on the characteristic spicules met with ; and, j' 

 though within very brief limits, ventures on a classifica- 

 tion, with a detailed nomenclature of these protean 

 forms. Whether the mass of the new names will meet 

 with acceptance or not, time must decide ; but there can 

 be no doubt that the author deserves every commenda- 

 tion for his brilliant effort to reduce the various forms into 

 an orderly sequence. Within the last few years the im- 

 provements in the methods of technique have opened new 

 vistas into the histology of Sponges. The classification 

 of the group is given in detail as far as the families : the 

 Phylum being regarded as derived from the choanoflagel- 

 late Infusoria, but pursuingfor a certain distance a course 

 of development parallel with that of the Metazoa. The ; 

 paragraph on the sponges of commerce is very interesting aBl 

 we do not know of an equally accurate account in English,^P 

 and yet no toilet article is in more universal use or more 

 talked about. The little that is known as to the extent 

 of the large trade in sponges may be judged of from the 

 fact that the latest information the author was able to get 

 on the subject dates from 1871, and that it only gives an 

 account of the sponges sold in Trieste for that year. This 



