I30 



NATURE 



yjtme 17, 1875 



devil, are so called from their predatory and fierce 

 nature. They have large canine teeth and sharp molars. 

 The second and third toes are no longer bound together, 

 whilst the great toe is absent or small. Myrmecobius is 

 a peculiar genus, remarkable for the great number of its 

 back teeth. The Tasmanian Wolf is confined to that 

 island, and will very probably soon become quite extinct, 

 because of its destructiveness to the sheep of the colonies. 

 It differs from all other members of the Kangaroo 

 order in that cartilages represent the marsupial bones 

 found in every other member of the order. The last family 

 consists of the true Opossums, which differ from all 

 above referred to in inhabiting America only, not Aus- 

 tralia. They are called Didelphidas ; one species is 

 aquatic in habit, and web-footed. 



Such are the verv varied forms composing the six 

 families which together make up the Kangaroo order. 

 "What is its relation to those of the other Mammalia ? 

 Very noticeable in it is the very great diversity of form, 

 dentition, and habit found in the order, some being 

 arboreal and vegetarian, others terrestrial and carni- 

 vorous, &c. ; nevertheless, these so varied marsupial 

 forms possess in common important characters by 

 which they differ from all other mammals. These 

 characters, however, relate mainly to the structure 

 of their reproductive organs, as to the great importance 

 of which characters naturalists are agreed. The angle 

 of the lower jaw is also peculiar. Almost every 

 mammal which has marsupial bones has the angle of its 

 jaw inflected, or else has no angle at all, whilst every 

 animal which has both marsupial bones and an inflected 

 jaw-sngle, possesses also those other special characters 

 which distinguish the marsupials from all other mammals. 

 We have, therefore, at least two great groups, one non- 

 marsupial, containing man, the apes, bats, cats, hoofed 

 beasts, &c. — the Monodelphia ; the other containing t'le 

 marsupials only — the Didelphia. There is a third group 

 containing only the Ornithorhynchus and Echidna, which 

 lorm by themselves alone a third group, Ornithodelphia. 



As to its zoological relations, we may therefore say that 

 the Kangaroo is a peculiarly modified form of a most 

 varied order of Mammalia (the marsupials), which differs 

 from all ordinary beasts (and from man) by very impor- 

 tant anatomical and physiological characters, the sign of 

 the existence of which is the coexistence in it of mar- 

 supial bones with an inflected angle of the lower jaw. As 

 to the geographical relations of the Kangaroo, a study of 

 their distribution over the world shows that the Kangaroo 

 is one of an order of animals confined to the Australian 

 region and America, the great bulk of the order, in- 

 cluding all the Macropodidse, being strictly confined to 

 the Australian region. 



The lecturer concluded by explaining the geological 

 relations of the Kangaroo and its order, pointing out that 

 in Australia we have an instance of zoological " survival " 

 connecting the existing creation with the triassic period. 



MA GNE TO-ELECTRIC MA CHINES * 

 II. 

 T N 1 871 M. Jamin communicated to the French Academy 

 ■*■ of Sciences a short note by M. Gramme, on a magneto- 

 electric machine which gave electrical currents always in 

 the same direction by the revolution of an electro-magnetic 

 ring between the poles of a permanent magnet. The 

 construction of the electro-magnetic or ring armature in 

 Gramme's machine differs in some mechanical details 

 from that of the transversal electro-magnet of Pacinotti, 

 and the serious mistake of applying the rubbers which 

 carry off the current at the wrong place is avoided. We 

 must therefore regard the Gramme machine as the first 



■dI?'^^ •r'^'^f-^^^ of a Lecture, with aUditions, delivered at the Belfast 

 Philosophical Society, March 17, by Dr. Andrews. F.R.S., L. & E, (Con- 

 iiuied frcm p. 92.) 



effective magneto-electric machine constructed to give 

 continuous currents all flowing in the same direction. 

 Before entering into the details of its construction it may 

 be useful, even at the risk of some repetition, to describe 

 as briefly as possible the principles on which the action 

 of the electro-magnet or ring armature depends. 



In its simplest form this armature consists of a ring of 

 soft iron, round which is wound a single closed coil of 

 copper wire or other metallic riband, covered with silk, 

 except at a single point in each loop of the coil, which is 

 left exposed in order to make contacts. In Fig. 4 such a 

 ring is shown, placed between the poles of a permanent 

 magnet. The parts of this ring contiguous to the poles 

 N s of the fixed magnet will acquire respectively polarity 

 of the opposite kind to that of the neighbouring pole, 

 while the parts of the ring o o', at the end of a diameter 



Fig. 4. — Ring Armature. 



at right angles to the line joining the poles, will be neutral. 

 If the ring; is made of homogeneous metal, this statement 

 will be strictly exact so long as it is at rest, but if it be 

 made to revolve rapidly on an axis perpendicular to the 

 plane of the fixed magnet, the poles of the ring, as well 

 as the neutral points, will be slightly displaced, as M. 

 Gaugain has shown, in the direction of the motion. This 

 arises from what is called the coercive power of iron; 

 that is, from the circumstance that even the purest iron 

 will not acquire or lose magnetism in an inappreciably 

 short period of time. The change in the distribution of 

 the magnetism in the ring from this cause is, however, 

 inconsiderable, and may easily be allowed for. 



To make the explanation clearer, let us suppose that 

 there is only one loop of wire, a (Fig. 5),''upon the ring, 

 and that this loop is moveable and in connection with a 

 galvanometer g. If now the ;loop:'is moved along the 

 ring (assumed to be at rest) from^ the neutral line O 

 towards s', a current will be developed in a certain direc- 



