TBANs ACTIONS OF SECTION A. 441 



for public discussion, aud, perhaps more -Naluable still, private interchange of 

 opinion ? Let us endeavour, one and all, to turn them to the best account. 



The following' Reports and Papers were read : — 



1. Second Report of the Committee for the Measurement of the Lunar 

 Disturbance ofGravitj. — See Reports, p. f5. 



2. Report of the Committee upon the present state of our Knowledge 

 of Spectrum Analysis. — See Reports, p. 120. 



3. On the Tension of Mercunj Vapours at Common Temperatures. 

 By Professor Lord Ratleigh, F.R.S. 



Tlie author called attention to the difficulty of reconciling- the values of 

 Eegnault and Ilagen with the phenomena observed by Crookes relating to the 

 viscosity of gases at high exhaustions. The total gaseous pressure in the working 

 chamber canuot be less than that of the mercury at the pump. If the penetration 

 of mercury vapour be prevented by chemical means, some other gas must be 

 present in equivalent quantity. If the value of Regnault and Ilagen is sub- 

 stantially correct, it does not appear how the phenomena coidd vary so much as 

 they are observed to do at the highest degrees of exhaustion as measm-ed by the 

 Macleod gauge. The question then arises whether the value of mercury tension 

 hitherto received may not be much in excess of the truth. In Ilagen's researches 

 it is assumed without reason that the pressure in a chamber of variable tempe- 

 rature is governed by the temperature of the coldest part, but this consideration 

 tells in the wrong direction. It was suggested that possibly a change in the 

 capillary constant, or currents in the fluid mercury at the chilled surface of the 

 meniscus, might have had something to do with the minute changes of level which 

 have been attributed to difl'erences of pressure in the mercury vapour. 



4. On the Yelocity of White and Coloured Light. 

 Bij Professor G. Forbes, M.A., F.R.S.E. 



The author gave an account of the experiments made by him in conjunction with 

 Dr. James Young, F.R.S. , with a view to determining the velocity of light. This 

 researcli has been published in the Transactions of the Royal Society. The chief 

 point of interest is that it appears that the velocity of blue light is greater than 

 that of red, the ditlerence being between 1 and 2 per cent, of the whole velocity. 



5. Preliminarn Account of Results obtained during the late total Solar 

 Eclipse {May 17, 1882). By Dr. Aethur Schuster, F.R.S., and 

 Captain Abney, F.R.S. 



Three photographs of the corona were obtained with different exposures. The 

 comet Tewfik, discovered during the eclipse, appears on the photographs, and the 

 change in its position on successive plates shows that it was receding from the sun. 

 A plate exposed in a camera, which had a prism in frout of the lens, showed a 

 series of impressions corresponding to the different kinds of light sent out by the 

 prominences. It is seen from this that diflerent prominences were at different 

 temperatures. The most intense image in every case corresponds to the calcium 

 lines. 



A photograph obtained in a complete spectroscope gives a complicated promi- 

 nence spectrum, a strong continuous spectrum in the lower parts of the corona, a 

 reversal of the solar line CI in the upper regions, and a great number of coronal 

 lines in the blue, violet, and ultra-violet. 



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