TRAXSACTIOXS OF SECTION A. .'',41 



furnishing information about the state of polarisation in the case of isotropic 

 media, with wliich the paper of which this is an abstract more particularly deals. 

 By an undulation is to be understood an uninterrupted train of waves which are 

 all alike — of the same wave-length intensity and state of polarisation, and each 

 undulation occupying the whole of space for all time. Accordingly, if the medium 

 be of limited extent, we must, for the purposes of this resolution, take into account 

 what would occur throughout an extension of the medium. At the same time 

 it is convenient in practical problems, and is legitimate, to consider separately 

 what happens within a prescribed space and at a definite time. The resolution of 

 a given distribution of light into its component undulations of flat wavelets is unique 

 in the same sense in which resolutions by Fourier's Theorem, or into Spherical 

 Harmonics, are unique. 



Department III. — Astronomy and Cosmical Physics. 

 The following Papers and Reports were read : — 



1 . Illustrations obtained by Photography of the Evolution of Stellar Systems. 

 By Isaac Roberts, D.Sc, F.R.S. 



2. Radiation in Meteorology. By W. N. Shaw, F.R.S. 



The purpose of the paper was to invite meteorological observers to make 

 special observations of the eHTect of radiation upon thermometers exposed under 

 diflerent meteorological conditions as regards cloud. The author explained that 

 the effect upon a cloud of gain or loss of heat by radiation was to lower or raise 

 the temperature of the cloud according to the temperature gradient, and pointed 

 out as a consequence that the specific heat of a cloud might be positive or negative, 

 and that for the particular case of adiabatic gradient the specific heat of air is 

 negative and indefinitely large. Hence it followed that the tracing of changes of 

 temperature by means of radiation observations might be of great assistance in 

 tracing the changes in the thickness of floating clouds. 



He then indicated the nature of the observations upon radiation which are 

 already included in the meteorological routine of a second order station, and sug- 

 gested the comparison of the black bulb with the simultaneous reading of the 

 screened dry bulb in various conditions of weather, and also pointed out some 

 unexplained phenomena connected with radiation which arose from a comparison 

 of the records of a photographic sunshine recorder with those of a Campbell- 

 Stokes instrument. 



3. On the Figure of the Earth. By Major S. G. Burrapd, R.E. 



Deflections of the plumb-line placed, and have continued to place, insuperable 

 obstacles in the way of correct determinations of the figure of the earth. The 

 pains we take in observing for latitude seem thrown away ; the correctness of our 

 result must depend on the direction of gravity, and at no place on the earth can 

 the direction of gravity be relied upon to indicate the true vertical. We could 

 measure the distance from Londonderry to Waterford within 2 or 3 feet ; we 

 could observe their latitudes within .5 or 6 feet ; but deflections of gravity at 

 Londonderry and Waterford might, without being extraordinary, throw us out by 

 800 feet. 



About 1860 Russian surveyors made the discovery at Moscow that deflections 

 of the plumb-line existed on flat unbroken plains ; that these deflections varied 

 sixteen seconds in 18 miles where no mountain was visible. This discovery 

 showed a local underground variation in the density of the earth's crust. It had 

 a profound influence on all subsequent discussion, and deflections of the plumb- 

 line were everywhere now readily attributed to local underground causes. But 



