May 2 1, 1908] 



NA TURE 



59 



and the other when strained by the various forces. The 

 strains are determined by measuring corresponding lengths 

 and angles on the two photographic negatives by means 

 of the optical projection micrometers which are exhibited. 

 The stresses are calculated from the measured strains by 



Fig. I. — India-rubber model and apparatus u^ed lu 

 stress in dams. 



the equations relating them, which have been experiment- 

 ally verified. 



Sir John Tliornycroft. F.R.S. : An instrument to indicate 

 the relative rate of turning of two bodies. A sphere sup- 

 ported on two equal revolving cylinders rotates on axes 

 in the same plane as the axes of the cylinders ; the angular 

 position of the axis of the sphere depends on the relative 

 velocities of the two cylinders, and is indicated by a hand 

 controlled by a roller touching the sphere. — Mr. C. V. 

 Boys, F.R.S. : (i) A modification of the well-known hypo- 

 cycloidal straight line motion of very simple construction, 

 and requiring guides only 2 \/2 (o-6 nearly) inch long for 

 a 4-inch stroke. (2) An artificial horizon in which the 

 mercury is spread out into a very thin film on a surface 

 which it does not wet, its edge being held bv deeper 

 mercury in a peripheral trough. The film is adjusted in 

 thickness by means of a floating plunger actuated by a 

 screw. Ripples set up by vibration die out almost in- 

 stantaneously, and altitudes of stars may bo taken in towns 

 where with deeper mercury that would be impossible. The 

 mercury is not contaminated by its dry supporting surface. 

 — Mr. H. G. King and Mr. R. Kerr: "Master gauges" 

 or " standards " for extremely accurate measurements, the 

 invention of Mr. C. E. Johansson, of Sweden. By using 

 these gauges separately or combined together, more than 

 80,000 different sizes can be obtained, any of which sizes 

 are accurate to within 0-00004 inch at 66° F. The steel 

 is so treated as to reduce to a minimum any chance of 

 change after being hardened. — Prof. H. L. Callendar, 

 F.R.S., and Prof. W. E. Dalby : Apparatus for measuring 

 temperatures in the cylinder of a gas engine. — Mr. Joseph 

 Goold : A uniformly symmetrical twin-elliptic pendulum. 

 As the deflector consists of a pair of cross-bars, its mass 

 can be virtually elongated by fi.xing the bars at any other 

 than a right angle. By this means any rate of change of 

 phase is easily secured. The resulting figures are made 

 available for stereoscopic efl'ects by taking two similar 

 figures and inverting one of them, so that each half figure 

 becomes associated (in the stereoscope) with the coinple- 

 mentary half of the other figure. — Mr. Charles E. Benham : 

 Stereoscopic effect of twin-elliptic figures. 



Cantbridrrc Scientific Instrument Co. : The Cambridge 

 patent extensometer. No mirrors or microscopes are used 

 for magnifying the movement, but the extension of the 



NO. 2012, VOL. 78] 



tesi-piece is measured by a micrometer screw and a 

 modified form of contact measurement. The instrument 

 is made in two separate pieces ; the lower piece carries the 

 micrometer screw shown in Fig. 2, and the upper piece 

 carries a spring tongue. These are fixed to the test-piece 

 by pressing the conical points of hard steel 

 rods into centre punch marks in the side of 

 the test-piece, these points being mounted in 

 strictly geometric slides. Both the upper and 

 lower pieces are held in the definite positions 

 shown in the illustration. If the test-piece 

 stretches, the upper piece rotates about the 

 conical points in the depressions in the test- 

 piece, and the end of the tongue approaches 

 the point on the micrometer screw head, the 

 upper piece forming a lever. The arms of 

 the lever are such that the part of the tongue 

 opposite the point on the micrometer head 

 moves five tinies the amount of the extension 

 of the test-piece. By means of an adjustment 

 the ratio of the arms of the lever can be 

 adjusted so that this multiplication of the ex- 

 tension can be made exact. One arm of the 

 lever is the flexible steel tongue which carries 

 a hardened steel knife-edge near its outer end. 

 If the tongue is bent sideways the knife-edge 

 is moved across the hard steel point, which 

 is carried from the centre of the divided head. 

 To adjust thi' instrument the screw is turned 

 and the point advanced until contact is made. 

 Relative movement of the lower piece carry- 

 ing the micrometer screw and the tongue is 

 thus measured, and is proportional to the 

 extension of the test-piece. It can, however, 

 initi...ii 01 be adjusted more accurately by causing the 



spring to vibrate and noting the soimd caused 

 by it touching the point each time it passes 

 over it. If the point is advanced by i/iooo mm. nearer 

 the screw the sound produced by contact as the spring 

 vibrates is louder, and the final adjustment of the micro- 

 meter screw can be made quickly and accurately. The 

 micrometer screw has a pitch of J mm., and the head is 

 divided into 100 parts ; each division on the head corre- 



FlG. 2. — Cambridge patent extensometer. 



sponds to an extension of i/iooo mm., and as the tenths 

 of divisions can be estimated by eye, readings can be 

 taken to 1/10,000 mm., although it is not claimed that the 

 results are trustworthy to this degree of accuracy. 



Mr. J. W. Gordon and Mr. H. Fletcher Moidton : A 



