March 31, 1887] 



NA rURE 



515 



motion, we immediately place upon the pUte a disk or a 

 coin, to which tha fingers give a first movement of rota- 

 tion round one of its diameters. 



The hand bein^ quickly drawn back, the aerial vortex 

 continues to make the coin turn round like a lop, and 

 absolutely keeps it captive in its radius of action. The 

 coin, while turning upon one of its diameters, makes a 

 sphere, and a later experiment will show that a revolving 

 sphere constitutes a centre of attraction. 



Fig. 3. — Eijuililn-iuii! of Rcvoh-iiii; Spheres. — A free 

 sphere keeps itself in equilibrium, and turns round another 

 sphere, to which a rapid movement of rotation is imparted. 



The apparatus consists of a pin, \, which is able to 

 turn in a support, and has a pulley, made to receive a 

 transmitted movement. Upon the pin .a is placed a 

 sphere, s, composed of from eight to ten flat circular 

 pieces (these may be either fiat disks, or disks cut into 

 a crescent shape ; it does not matter which). The pin 

 may be at any angle whatever to the horizon ; in this 

 experiment it is inclined at 45', but it may be horizontal 



or vertical. The angle of 45' was chosen because it 

 seemed to be most difficult for the experiment, which 

 would therefore be the more conclusive. When the 

 sphere s is turned round rapidly, you feel on the 

 hand a strong blast which escapes all around from its 

 equator. Bits of piper which arc placed near it are 

 thrown far away. Nevertheless, if a balloon is put near 

 this blast, it is quickly attracted towards the revolving 

 sphere, and describes orbits round it in the plane of the 

 equator. .-\s the experiment took place in a room, 

 where there were obstacles producing eddies, and 

 as also gravity has too much effect by reason of the 

 proximity of the ground, it is very difficult to obtain a 

 regular movement. The balloon conies easily in contact 

 with the revolving sphere, and is then driven away by the 

 collision too far to bt caught again. A very simple con- 

 trivance consists in placing round the sphere s a wire 

 guard or circle of iron, I', i millimetre in diameter, 

 attached to the support by three similar wires. 



The b.illoon will then ke.^p on turning round the motive 



sphere, even ceasing to touch the guard in the lower part 

 under the action of gravity. The experiment can be made 

 in diBerent ways, and the guard may e\en be suppressed, 

 but these variations teach us nothing new. 



In studying the vortex movements which the sphere 

 imparts in the medium in which it is plunged, we easily 

 calculate the ratio of the attraction which it exercises on 

 the balloon. 



Fig. 4. — The guard of the revolving sphere is taken 

 away, and we place parallel to its equator a circle of paper 

 with an interior diameter greater than the exterior dia- 

 meter of the sphere ; the circle is caught into the move- 

 ment of rotation, and maintains itself strongly in the plane 

 of the equator. 



ON OLDHAM I A 



THE organic origin ofOldhamia has often been disputed. 

 Originally described by Edward Forbes from speci- 

 mens found at Bray Head, near Dublin, in rocks of the 

 Cambrian formation, it has been found in a few other locali- 

 ties in Ireland, in rocks of a similar formation. In the 

 dispute the weight of the evidence has seemed to be in 

 favour of the views of Forbes, Jukes, Harkness, and 

 Kinahan, that this lowly form is a fossil probably belong- 

 ing to the Polyzoa, or to the Sertularian Polyps. So the 

 matter has stood for a long time. A recent paper by 

 Prof W. J. Sollas, published in the Proceedings of the 

 Royal Dublin Society for January last, once again opens 

 the discussion. In the hope of throwing light on this 

 problematical structure, thin slices for microscopic ex- 

 amination were cut, both parallel and transverse to its 

 planes of cleavage. When these were placed under the 

 microscope, all trace of the Oldhamia structure appeared 

 to have vanished. An examination with the unaided eye 

 showed, however, that it was still there, presenting itself 

 as narrow, undulating, and branching bands of a lighter 

 colour than the surrounding matrix. Its appearance, 

 moreover, varied in an extraordinary manner according to 

 the direction in which it was viewed. Looked at obliquely 

 m a strong light, the thread-like bands are brilliantly 

 illuminated, and appear fiintly coloured with spectral 

 tints ; looked at directly, the bands become fainter, and 

 are less clearly distinguishable from the matrix. In cer- 

 tain positions the slice taken at right angles to the bed- 

 ding has an appearance somewhat suggestive of shot-silk, 

 and, from the planes of cleavage, markings which remotely 

 resemble in form the dendritic markings of Sutton stones 

 extend into the surrounding matrix. 



These appearances suggest the presence of some mine- 

 ral possessing high reflection or refraction arranged in 

 more or less parallel planes. Mr. Teall, in the same 

 paper, gives full details of the mineral characters of 

 the rock. Aided by these, Dr. Sollas finds that the 

 lighter-coloured bands, which correspond to the Oldhamia 

 markings, owe their distinction from the surrounding 

 matrix to the presence of an excess of sericite scales ; 

 and that the curious shot-silk appearances are produced 

 by the local deflections of these scales from parallelism 

 with the cleavage planes into directions tangential to 

 curves, which are probably transverse sections of those 

 long ridges which, when seen on the exposed surface of 

 a cleavage plane, are recognised as the usual form of 

 Oldhamia. It would appear possible that these ridges 

 are wrinklings of the cleavage planes produced during 

 the shearing which led to their formation. These obser- 

 vations were made on the form known as O. radiata, and 

 in some supplementary remarks Dr. Sollas adds that 

 when Oldhamia is present it shows itself on the surface 

 of the laminx as rounded discontinuous ridges, which 

 are without definite boundaries, and have the appearance 

 of fine wrinkles. When the phyllode is fractured obliquely 

 to the cleavage laminas, the Oldhamia markings are found 

 to extend through the rock as fine ridges or wrinkles 



