June 5, 1919] 



NATURE 



275 



THE ROYAL SOCIETY CONVERSAZIONE. 

 PREVIOUS to the war the Royal Society held 

 ■*• two conversaziones annually— one to which 

 j^entlomen only were invited, and the other at which 

 ladies as well as gentlemen were present. These 

 >ocial meetings were resumed on May 28, when a 

 distinguished gathering of men of science and others 

 met at Burlington House for the usual first conversa- 

 zione, after an interval of four years. Many exhibits 

 of apparatus and objects of scientific interest were 

 on view, and the subjoined descriptions of them are 

 abridged from the official catalogue. Exhibits re- 

 lating to like departments of scientific activity have 

 been brought together, and only such descriptions 

 have been included as can be comprehended without 

 seeing the actual objects. 



Prof. H. F. Newall: Dr. G. E. Hale's photographs 

 of the Zeeman effect in the spectra of sun-spots. An 

 image of the sun's disc is thrown by means of the 

 150-ft. tower telescope at Mount Wilson on the slit- 

 plate of the 75-ft. spectrograph. Close to the slit a 

 Xicol prism is placed. Above the Nicol prism are 

 mounted strips of mica 2 mm. wide, with their axes 

 inclined +45° and —45° to the length of the strips, 

 alternating in adjacent strips. This device is called 

 a compound quarter-wave plate. When a sun-spot 

 near the centre of the sun's disc falls on the slit 

 through the polariscopic apparatus, certain lines in 

 the spectrum are widened, and others resolved into 

 two or three components. From a comparison of 

 the solar effects and of the magnitude of the Zeeman 

 effect in experiments in the laboratory on the corre- 

 sponding lines, the strength of the magnetic field in 

 the sun-spot is deduced. Average spots exhibit fields 

 ranging from 2000 to 2700 gausses. The field varies 

 approximately in proportion to the size of the umbra. 

 Sir y'apier Shaw : Illustrations of the structure of 

 the atmosphere on selected occasions, (i) Records 

 of wind, on tube-anemometers, corrected for the 

 I difference of exposure in different orientations. 

 (2) Maos of stream-function of the air for different 

 levels on the occasion of the destruction of a fleet 

 of Zeppelins, October 19-20, 19 17, and another similar 

 distribution on October 13, igi8. (3) Theoretical 

 maps of the stream-function of the free air and dis- 

 tribution of pressure in the case of a cyclone consist- 

 ing of a simple vortex with maximum velocity 

 43 metres per second, enclosing a core of fluid- 

 rotating-like-a-solid, in a uniform atmospheric cur- 

 rent of 16 metres per second; with maps for iSh., 

 September 10, 1903, for comparison. 



Mr. George H. Gabb : Portrait of Dr. John Jeffries, 

 in pastel, by John Russel, R.A. Dr. Jeffries was, 

 with Blanchard, the first to cross the Channel in a 

 balloon, on January 7, 1785. The account of this 

 epoch-making " aerial voyage " was read before the 

 Royal Society in January, 1786. This portrait was 

 exhibited in the Royal Academy in 1786, and was lost 

 for more than a hundred years until it was discovered 

 a short time ago, quite unknown, among a miscel- 

 I laneous collection of pictures. Dr. Jeffries was the 

 I first to make an -ascent solely for scientific purposes, 

 and the first to attempt meteorological observations 

 from a balloon. In his ascent from London on 

 November 30, 1784, he included in his scientific equip- 

 ment a barometer, a thermometer, a hygrometer, an 

 electrometer, a marine compass, a telescope, and six 

 small phials filled with water given him by Caven- 

 dish in order to collect samples of air at different 

 altitudes. 



Prof. MacGregor-Morris : Portable apparatus for 



measuring air-currents. A Wheatstone bridge 



is made of four wires all exactly alike of a 



material the resistivity of which varies with tem- 



NO. 2588, VOL. 103] 



perature. This bridge is heated by the passage of 

 an electric current. Adjacent arms are so arranged 

 as to be unequally cooled when placed in an air- 

 current. The apparatus can be carried on a bicycle, 

 and has been used for determining the velocity of the 

 wind about a cliff-edge, and also around the gallery 

 of a lighthouse. 



Royal Aircraft Estahlishment. Farnhorough: 

 Standard and research aeronautical instruments. 

 (i) R.A.E. Mark II. Compass. — An instrument de- 

 signed bv the late Capt. Keith Lucas to avoid, so 

 far as possible, the errors which occur when flying 

 in a northerly course. (2) R.A.E. Accelerometer. — 

 This instrument records the variations of apparent 

 gravity on an aeroplane by photographing the move- 

 ments of a fine glass fibre bent into a- bow. (3) 

 R.A.E. Kymograph. — ^The instrument is to record 

 movements of the aeroplane in roll, pitch, or yaw. 

 (4) R.A.E. Pressure-plotting .Apparatus.— The ap- 

 paratus records the pressure or suction over an aero- 

 plane's wings bv means of small pipes which open 

 flush with the surface and lead to a multiple recording 

 pressure-gauge. (5) R.A.E. Climbmeter.— An instru- 

 ment which indicates the rate at which an aeroplane 

 is rising or falling. 



The Cambridge Scientific Instrument Co., Ltd.: 

 Dr. G. \. Shakespear's katharometer for measuring 

 the puritv of gases. Two small spirals of platinum 

 wire form two arms of a Wheatstone bridge, and 

 their resistances, depending on their temperatures, 

 depend on the viscosities of the surrounding gases. 

 .\ galvanometer connected across the bridge indicates 

 its out-of-balance, and is calibrated to give a direct 

 reading of the purity of the gas. or otherwise, as 

 required. Manv practical applications are possible: 

 (a) A hvdrogen puritv meter for use with aircraft is 

 exhibited; (b) permea'meters for testing airship fabrics 

 and exploring seams or searching for leaks are ex- 

 hibited; and (c) a humidity recorder showing the 

 vapour pressure in the air of the exhibition room was 

 shown working. 



Mr. F. W. Aston: Rapidly moving striated dis- 

 charge in neon and helium. The light in the capillary 

 of a spectrum discharge tube containing neon or 

 helium is apparently continuous, but when analysed 

 bv a rotating mirror is found to consist of a proces- 

 sion of alternate bright and dark bands or striations 

 travelling in the direction of the current, i.e. from 

 anode to cathode. These appear in the mirror as 

 ribbons of light, their waviness indicating variations 

 in speed and being more marked in neon than in 

 helium. The mean velocity can be calculated from 

 the slope, and is found to be approximately that of 

 pressure-wave propagation, i.e. sound, in the gas in 

 the discharge tube. 



Mr. C. T. R. Wilson: (i) Stereoscopic photographs 

 of the tracks of ionising particles through air. By 

 causing water to condense upon the ions set free, the 

 invisible trail of ions left by each flying particle 

 along its course is converted into a sharplv defined 

 line of cloud. Stereoscopic photographs of the tracks 

 thus rendered visible are taken before convection 

 currents have had time to distort them. (2) Photo- 

 graphic record of the changes in the electric potential 

 gradient during a thunderstorm. The record showed 

 the sudden changes produced in the electric field by 

 the passage of lightning discharges. 



Prof. E. H. Barton and Miss H. M. Browning: 

 Vibrations, forced and coupled. The phenomena of 

 forced vibrations and resonance were experimentally 

 illustrated by a number of pendulums of graduated 

 lengths, with light bobs hanging from . a horizontal 

 cord and «5et vibrating bv a pendulum, with heavy 

 bobs hanging from the same cord. All the salient 

 points of the mathematical theory of' forced vibrations 



