March 20, 1913] 



NATURE 



59 



tically certain that the ruminants represented in the 

 sculpture are antelopes. They must, moreover, be 

 antelopes of an African type, as there are no marsh- 

 haunting species with spiral horns known from Syria, 

 or Asia in general, and the presumption is that they 

 represent an extinct member of the tragelaphine group 

 allied to the nyala and situtunga, in which the 

 females are hornless. The tragelaphine group is re- 

 presented at the present day in India by the nilgai 

 and chousingha, in which the horns of the bucks are 

 small, but there is evidence that in the Pliocene India 

 was the home of species akin to the kudu and bush- 

 buck. And it is therefore quite reasonable to expect 

 that in Assyrian times a member of the group mav 

 have inhabited the Euphrates Valley. 



R. Lydekker. 



Cavities in Stones. 



In the description of the Agglestone "on the old 

 moor of Studland, near the north shore of the Island 

 of Purbeck," given in Warne's "Ancient Dorset," 

 allusion is made to superficial cavities or hollows in this 

 stone, and in stones in Yorkshire and Lancashire. In 

 some cases " the cavities consist of holes about an inch 

 and a half broad and of the same depth drilled into 

 the stone." Mitchell ' gives illustrations of the stones 

 with cup-shaped markings described by Sir James 

 Simpson in his work on " Archaic Sculptures." 



In all probability these examples of supposed archaic 

 sculptures (and others) have long ago received the 

 "more extended investigation by competent observers" 

 that Warne thought they deserved. But it would be 

 of interest to know if they have been examined 

 by conchologists as well as archaeologists. There 

 is just a possibility that some may be burrows 

 excavated by Helix aspersa, for the description and 

 illustrations recall the helicidean cavities in Carbon- 

 iferous Limestone that occur somewhat frequently in 

 Ireland, but are uncommon in Britain. The rock- 

 shelters of Helix aspersa at Great Orme's Head, 

 Llandudno, and at Miller's Dale, Derbyshire, have been 

 fully described and illustrated, 2 also others more re- 

 cently observed by myself in the limestone on Brean 

 Down, Weston-super-Mare. 3 . 



E. W. S WANTON. 



Sir Jonathan Hutchinson's Educational Museum, 

 Haslemere, March 10. 



An Experiment for Showing Lines of Force in an 

 Electrostatic Field. 



A gilt cork ball, about i cm. in diameter, is 

 attached by sulphur to a vertical straw about 28 cm. 

 in length. The lower end of the straw is fastened by 

 sulphur to the centre of a circular cardboard tray 

 about 5 cm. in diameter, in which is a ring of lead. 

 The tray is put on a watch glass which floats on the 

 surface of mercury in a large flat dish. (A developing 

 dish about 30 cm. by 26 cm. was used, but a shallow- 

 wooden trough made for the purpose would be better.) 

 In this way the gilt ball is able to move fairly freely 

 in a horizontal plane. This float arrangement is kept 

 in a bell-jar desiccator when not in use. 



Two conducting spheres, about 10 cm. in diameter, 

 are mounted on vertical glass tubes (sealed off at each 

 end), and coated for about 10 cm. with sulphur, which 



1 "The Past in the Present." p. 86. 



2 John Taylor, " Monograph of the T and and Freshwater Mollnsca of the 

 British Isles," vol. i., p. qu, fie. 601, and vol. iii., pp. 214-246. 



'■'■ E. W. Swantnn. "The Mollnsca of Somerset " (Somerset Arch, and 

 Nat. Hist. Soc, 1912), pp. 26, 27, pi. iii. 



can be readily got into a good insulating condition 

 when required by warming in a flame. The centres 

 of these spheres and the gilt ball are at the same 

 level. The spheres being arranged on opposite sides 

 of the dish, and so that the ball can touch them. 



The spheres are connected either to the same ter- 

 minal or to the opposite terminals of a Wimshurst 

 machine. 



The gilt ball describes curves which, when it moves 

 slowly, give the general directions of the lines of force 

 between the spheres in the plane it is free to move in. 



The experiment is effective for illustrating lines of 

 force in an electrostatic field and for leading up to 

 the mathematical definition of potential. It mav be 

 extended for different charges on the spheres. 



R. F. D'Arcy. 



Caius College, Cambridge. 



Units of Pressure in Vacuum Work. 



Surely physicists do, or should, for convenience, 

 always express wave-lengths in microns (/*) and mole- 

 cular distances in millimicrons (/»m). Why not follow 

 the same practice in dealing with vacua? The milli- 

 metre is a convenient unit down to, say, o-i mm., but 

 1/1000 mm. and 1/10,000 mm. have frequently to be 

 expressed. It is simpler to write and comprehend 

 these in the form i p- or o-i m. Again, in the pamphlet 

 sent out by Dr. Gaede to describe his very successful 

 pumps, we see unwieldy decimal expressions used. 

 For instance, it is stated that it is possible to obtain 

 a pressure of 0-000002 mm. of mercury after four 

 minutes of pumping. Why not write this 2nn of 

 mercury ? 



There is a small unit sometimes found in researches, 

 viz. one-millionth of an atmosphere, denoted by the 

 letter M, but for this unit to have a definite numerical 

 meaning it is necessary to quote the barometric read- 

 ing at the time. If the barometric reading is normal 

 1 M = o-76 f-. But, of course, 076 p- alone needs no 

 qualifying as to the barometric pressure, and therefore 

 is simpler and more direct. P. E. Shaw. 



University College, Nottingham, March 2. 



NEW MICROSCOPE EYEPIECES. 



Eyepiece Micrometer. 



T^\R. METZ, one of the researchers employed 

 -L^' in the Leitz optical factory at Wetzlar, has 

 recently described * a micrometer for use with the 

 microscope which, if we are not mistaken, will 

 rapidly replace all others, including the expensive 

 filar micrometer where a mechanical stage is avail- 

 able. The root idea is that the scale used is such 

 that microns can at once be read off without 

 greatly changing the tube-length, or considering 

 the micrometer value of the objective employed, 

 and therefore dispensing with the arithmetic for 

 which this is a necessary datum. 



To bring this about, the intervals of the new- 

 scale, instead of being 1/10 01 1/20 mm. wide, as 

 is usually the case in eyepiece micrometers, have 

 a definite value of o'o6 mm. 



With an objective of 2 mm (,'„) focus when a 

 stage micrometer with ten j— mm. divisions is 

 viewed, each of these divisions falls on the larger 



NO. 2264, VOL. 9l"| 



1 Zeit.JUr 



• :nschaftUcJie Miknnkopic, : 



