476 Royal Society. 



of luminous gas existing in masses which are probably continuous ; 

 and the nebulae in the telescope present not points, but surfaces, 

 in some cases, subtending a considerable angle. As long as an object 

 remains of sensible size in the telescope it retains its original bright- 

 ness, except as this may be diminished by a possible power of extinc- 

 tion belonging to celestial space, and by the absorptive power of the 

 earth's atmosphere. 



By means of a special apparatus the light of three nebulae was 



compared with the light emitted by a sperm candle, burning at the 



rate of 158 grs. perhour. The results are that — [candle. 



The intensity of nebula, No. 4628 1 H. IV. = T J r? th part of that of the 



„ „ annular nebula in Lyra = 6 ^ 2 nd „ ,, 



„ „ Dumb-bell nebula —t^Vut^ 1 » » 



The estimation in each case refers to the brightest part of the 

 nebula. The amounts are too small by the unknown corrections for 

 the loss which the light has sustained in its passage through space 

 and through the earth's atmosphere. These values have an impor- 

 tance in connexion with the gaseous nature of the source of the light, 

 which the spectroscope indicates. Similar estimations made at con- 

 siderable intervals of time might show whether the brightness of 

 these bodies is undergoing increase, diminution, or a periodic variation. 



The paper concludes with some observations on the measures of 

 the diameters of some of the planetary nebulas. A very careful set 

 of measures of 4232, 5 2, by the Rev. W. R. Dawes, F.R.S., is 

 given, which makes the equatorial diameter= 15"* 9. Also measures 

 by the author of 1414, 73 H. IV. which give its diameter in R. 

 A.=30"'8. 



February 22. — J. P. Gassiot, Esq., Vice-President, in the Chair. 



The following communication was read : — 



" Account of Experiments on the Flexural and Torsional Rigidity 

 of a Glass Rod, leading to the Determination of the Rigidity of 

 Glass." By Joseph D. Everett, D.C.L., Assistant to the Professor 

 of Mathematics in the University of Glasgow. 



In these experiments a cylindrical rod of glass is subjected to a 

 bending couple of known moment, applied near its ends. The 

 amount of bending produced in the central portion of the rod is 

 measured by means of two mirrors, rigidly attached to the rod at 

 distances of several diameters from each end, which form by reflexion 

 upon a screen two images of a fine wire placed in front of a lamp- 

 flame. The separation or approach of these two images, which 

 takes place on applying the bending couple, serves to determine the 

 amount of flexure. 



In like manner, when a twisting couple is applied, the separa- 

 tion or approach of the images serves to determine the amount of 

 torsion. 



The flexural and torsional rigidities, / and t, which are thus found 

 by experiment, lead to the determination of Young's Modulus of 

 Elasticity, M (or the resistance to longitudinal extension), and the 

 absolute rigidity, n (or resistance to shearing), M being equal toy 

 divided by the moment of inertia of a circular section of the rod 



