386 Royal Society : — 



nary sunlight, becomes homogeneously tinted, the author appends 

 the readings, taken in the way described, from various portions of a 

 piece of the standard paper used for the sun-pictures exposed for 

 some seconds to direct sunlight. 





Reading. 



Deviation from mean 



ion No. 1 . . 



101-4 



+ 0-93 



2 .. 



1007 



+ 0-23 



3 .. 



98-5 



-1-97 



„ 4 .. 



101-6 



+ 1-13 



5 .. 



99*9 



-0-57 



6 .. 



1007 



+ 0-23 



Mean . . 



100-47 





The sun-pictures obtained on the sensitive paper must possess 

 only a slight tint, otherwise the differences in shade cannot be 

 accurately observed ; they then exhibit a peculiar coarse mottled 

 appearance, which is not due to imperfections in the paper or the 

 lenses, nor to the action of the earth's atmosphere. 



Perhaps these irregular dark and light patches are owing to clouds 

 in the solar atmosphere, and they may have an intimate connexion 

 with the well-known phenomenon of the red prominences. 



Mr. Baxendell and the author propose to carry out, according to 

 this method, a regular series of observations of the variation of the 

 relative amounts of brightness on the sun's disc, and they hope be- 

 fore long to be able to present the Society with some further details. 



March 17, 1864. — Major-General Sabine, President, in the Chair. 



The following communication was read : — 



" Description of an Improved Mercurial Barometer." By James 

 Hicks, Esq. 



Having shown this instrument toMr.Gassiot,he wished me to write 

 a short description of it, which he thought would be of interest to the 

 Boyal Society. 



Some time since I constructed an open-scale barometer, with a 

 column of mercury placed in a glass tube hermetically sealed at the 

 top, and perfectly open at the bottom. The lower half of the tube 

 is of larger bore than that of the upper. 



If a column of mercury, of exactly the length which the atmo- 

 sphere is able at the time to support, were placed in a tube of glass 

 hermetically sealed at the top, of equal bore from end to end, the 

 mercury would be held in suspension ; but immediately the pressure 

 of the atmosphere increased, the mercury would rise towards the top 

 of the tube, and remain there till, on the pressure decreasing, it 

 would fall towards the bottom, and that portion which the atmo- 

 sphere was unable to support would drop out. But if the lower half 

 of the tube be made a little larger in the bore than the upper, when 

 the column falls, the upper portion passes out of the smaller part of 

 the tube into the larger, and owing to the greater capacity of the 

 latter, the lower end of the column of mercury does not sink to the 



