562 



NA rURE 



[April 29, 1922 



The Limbs of Trilobites. — Those zoologists and 

 palaeontologists who have been fortunate enough to 

 look through P. E. Raymond's recent monograph on 

 the trilobites (see Nature, vol. io8, p. 481) will be 

 glad to note Dr. C. D. Walcott's additions, comments, 

 and suggestions as to limb-structure, in his recent 

 paper on the Cambrian Neolenus (Smithsonian Misc. 

 Coll., vol. 67, No. 7, 192 1). The author proposes to 

 put forward the results of further investigations into 

 the organisation of trilobites " in the course of two 

 or three years," so that our anticipations when 

 directing attention to the stimulating nature of 

 Raymond's paper were evidently correct. 



Geology for Townsmen. — The Geological Survey 

 of Great Britain has done great service to those who 

 dwellor work in London by issuing a memoir by 

 H. Dewey and C. C. A. Bromehead on " The Geology 

 of South London" (Ordnance Survey, Southampton, 

 1921, price 3s. 6i.). This describes the country 

 covered by the recent colour-printed map, showing 

 superficial deposits. Sheet 270, on the scale of 

 I : 63,360. Starting from the gravels of Ealing, 

 Hyde Park, and Millwall on the north, we can realise, 

 as in a series of pictures, the broad exposures of 

 London clay south of the Thames, supporting at 

 Wimbledon outliers of Glacial gravel ; and in the 

 south we reach the chalk at Sutton, and rise to the 

 unspoiled uplands of Banstead Downs and Sander- 

 stead. The geological colouring well marks out the 

 old valley, widening quickly on the clay, by which 

 the railways now reach Croydon from the south. 

 The memoir contains a view of the area in 1794, 

 looking across London from the north, and another 

 of Richmond at about the same date. The sources 

 of these should have been mentioned ; they appear 

 to be from separately published plates. A city 

 and suburban map, of equal interest, but of a very 

 different nature, is the Johannesburg Sheet (52) of 

 the Geological Survey of South Africa. The scale 

 is near i : 150,000. With the attached longitudinal 

 section, it is an admirable exposition of the geological 

 relations of the Witwatersrand system, and the 

 moderate price of 5s. includes a memoir by C. T. 

 Mellor and A. L. Du Toit (Pretoria, 1921). The area 

 covers the whole historic mining district, and extends 

 across the dolomite to the reappearance of the Rand 

 rocks at Heidelberg. 



Electrification of Phosphorus Smoke Nuclei. 

 — ^With reference to a note on electrical precipitation 

 which appeared in Nature of March 23, p. 388, we 

 have received a paper by J. J. Dowling and C. J. 

 Haughey (Proceedings of the Royal Irish Academy, 

 vol. 36, Section A, No. 3) on the electrification of 

 phosphorus smoke nuclei. The authors' experiments 

 indicate that the nuclei in the smoke of phosphorus 

 slowly oxidising in air acquire charges in an electric 

 field, that the charges depend on the strength of 

 the field, and that they vary in whole multiples of 

 the electron. Charges from one to twenty-five 

 electrons were observed. They also indicate that 

 when large numbers of particles, of nearly equal 

 sizes and charged equally, are exposed to the same 

 electric field, the particles may be dragged through 

 a comparatively narrow channel in the air, and may 

 carry the air in this channel along with them. The 

 mobilities of particles in fumes exposed to electric 

 fields may, therefore, exceed considerably what 

 might be expected for the limiting velocity according 

 to Stokes's law applied to isolated particles (as in 

 the well-known experiments of Millikan). This effect 

 is obviously of importance in precipitation appar- 

 atus, and careful quantitative experiments of this 

 kind should be of assistance in the design of such 

 apparatus. 



NO. 2739, VOL. 109] 



A New Science Microscope. — We have received 

 from Mr. C. Baker of 244 High Holborn, W.C.i, an 

 example of his new Science Microscope which has been 

 specially designed to meet the requirements in science 

 schools (Fig. i). The stand is a single casting with a 

 foot of the horse-shoe type but with a posterior limb 

 which gives complete stability. 

 A large square stage, 4-2 in. 

 X4-2 in. is provided with 

 mirror below, with plain and 

 concave surfaces. The coarse 

 adjustment, of the diagonal 

 rack and pinion type, gives 

 a motion slow enough to en- 

 able the user to focus with 

 comfort a ^-in. objective — 

 actually we found that even a 

 xV-in. objective could be used 

 without difficulty. A com- 

 bination I in. and \ in. ob- 

 jective was supplied and both 

 powers were found to give 

 excellent definition and flat 

 field. The instrument is 

 well finished, rigid in all ^"" '" 



parts and strongly built, and should prove a very 

 serviceable one capable of withstanding rough 

 usage. For the stand alone the price is 4/. los., or 

 with a No. 3 eyepiece and combination i in., J in. and 

 \ in. objective, 6/. 17s. 61^. In this particular instru- 

 ment, the objective thread did not seem to be exactly 

 the R.M.S. standard size, for we were unable satis- 

 factorily to attach a Zeiss, a Leitz and four Swift 

 lenses, which themselves were completely inter- 

 changeable on their respective instruments. 



Thermometers for measuring Rock Tempera- 

 tures. — An interesting new type of thermometer has 

 recently been designed and constructed by Messrs. 

 Negretti and Zambra, for the purpose of measuring 

 the surface temperature of rocks, sand, or other 

 materials. Since the difficulty in design lies in 

 securing that the bulb shall take up accurately the 

 temperature of the surface, whatever the other 

 attendant circumstances may be, the new features of 

 the instrument mentioned are associated with the 

 bulb end of the thermometer. The other end of the 

 thermometer is simply supported so that the stem, 

 which is about 15 cm. long, lies parallel to the surface 

 and about 0-5 cm. from it. At the bulb end the stem 

 is curved so that the bulb (of rather larger size than 

 in a clinical thermometer) lies along and almost 

 touching the surface. Actual contact is effected by a, 

 thin flat copper plate lying on the surface and attached 

 to the bulb, which itself is copper-plated on the out- 

 side. The under surface of the plate is roughened, 

 and its relatively large surface (it is about i cm. wide) 

 quickly takes up the temperature of the rock by 

 conduction and radiation, and conducts the heat 

 rapidly to the glass bulb. The upper surface of the 

 plate, and the plated surface of the bulb, are polished 

 to prevent radiation to or from the surrounding 

 objects. The bulb is further insulated by a polished 

 reflector which covers it almost entirely, and prevents 

 radiation while also reducing conduction of heat by 

 air currents. The reflector is gilt in order that it shall 

 not become tarnished. The mass of the bulb is 

 small, so that it possesses little thermal capacity and 

 scarcely disturbs the condition of temperature existing 

 before placing the bulb on the rock ; the thermal 

 inertia is also small for the same reason. The makers 

 state that on holding the bulb near to the side of the 

 radiator with the polished surface facing the radiator, 

 the thermometer shows no change of reading, but on 

 turning it round so that the flat copper plate faces 

 the radiator, the column at once begins to move. 



