668 REPOET— 1882. 



allow the mean pressure at either end of the cylinder to be read off' by the use of a 

 Bourdon pressiu'e-gaucre. A pressure-gauge is mounted at each end of the cylinder, 

 the pipe leading to the gauge being throttled so as to allow a small entry and 

 exit of steam at each stroke. This throttling causes the pointer of the gauge to 

 rise gradually to the mean pressure, above or below which it makes small oscilla- 

 tions. To prevent condensation in the throttle or tube, these are kept hot by the 

 flame of a lamp placed below. ^ 



8. On a new form of Arc Laviv. 

 By Professor G. Foebes, M.A., F.E.S.E. 



This is a focussing arc lamp. Tlie carriers of the two carbons are supported by 

 vertical racks which are worked by pinions, one of which is twice the diameter of 

 the other. These pinions are on the same axis as the ring of a small Gramme 

 motor ; a current is supplied to this ring by a shunt from one on the main wires. 

 One field magnet of the motor is wound with thick wire and is in the main circuit ; 

 the other is wound with fine wire. It is connected as a shunt on the arc. 



9. On the Laws defining the Strength of Current ivhich can he sent through 

 Wires of different diameters luiihout raising the external tem]ieratureahove 

 a certain limit. By Professor G. Forbes, M.A., F.E.S.E. 



Law I. — When the wire is bare and exposed to the air, the strength of current 

 is more nearly proportional to the diameter of the wire than to the theoretical 

 value of the diameter raised to the power |. 



Laiv II. — When the wire is wound in coils of the same size and weight, the 

 strength of current is proportional to the square of the diameter of the wire. 



To find out the first law, a trace of wax was put upon the wires, and the 

 current necessary to melt it was measured. 



To find out the second law, thermometers were inserted in coils of equal size 

 and weight, and the currents required to raise each to the same temperature were 

 compared. 



TUESDAY, AUGUST 29. 

 The following Report and Papers were read : — 

 1. Report of the Committee on Wind Pressure. — See Reports, p. 315. 



2. On the Mechanical Properties of Aluminium. 

 By W. H. Barlow, F.Ii.S. 



A bar of aluminium, three feet long and a quarter of an inch square, was 

 obtained, and diflerent parts of this bar were subjected to test for tension, com- 

 pression, and transverse strain ; and also to ascertain the modulus of elasticity, the 

 elastic range, and ductility. 



It wiU be seen on reference to the results obtained, that the weight of a cubic 

 inch is '0275 lbs., showing a specific gravity of 2688, and that its ultimate tensile 

 strength is about 12 tons per square inch. The range of elasticity is large, the 

 extension at the yielding point being —-^ part of its length. The modulus of elas- 

 ticity is 10,000. The ductility in samples 2 inches long was 2'5 per cent. 



Taking the tensile strength of this metal in relation to its weight, it shows a 

 high mechanical value. Its characteristics in this respect, as compared with those 

 of other well-known metals, are shown in the following summary : — 



' H^ectrical Review, vol. xi, p. 197. 



