602 TRANSACTIONS OF SECTION A, 



thickness of whose walls was about 0-1 mm., aud the charge reaching a thick 

 insulated brass cylinder surrounding the glass tube per second was measured by a 

 quadrant electrometer. The whole was enclosed in a silvered glass vessel which 

 could be exhausted by means of charcoal and liquid air. In order to allow for 

 the absorption of the /3 rays from the emanation by the walls of the enclosing 

 glass tube, a series of experiments was made to determine the law of absorption 

 of the |3 rays from radium B and C when the thickness of glass is varied. It 

 was found that the law of absorption for glass was sensibly the same as for 

 aluminium as found by 11. W. Schmidt, the radiation being measured in both 

 cases by the ionisation produced by the rays after traversing different thicknesses 

 of glass. It was further found that the law of absorption was the same when 

 measured by the charge received by the brass cylinder mentioned above when 

 different thicknesses of glass were interposed between the emanation and the 

 brass cylinder. It is therefore concluded that when rays pass through matter 

 the observed absorption of the rays is not due to scattering, but to an actual 

 stoppage of the particles by the absorbing medium. The final corrected value for 

 the number of 3 particles emitted per second from the radium C in equilibrium 

 with 1 gram of radium is 4"9 !< I0'°, 



8, The Rate of Production of Helium from Radium, 

 By Sir James Dewab, FM,S. 



Depaetment of Cosmical PaYSica (Astronomy). 

 The following Report aud Papers were read : — 

 1. Report oj the Seismological Committee, — See Reports, p. 60, 



2. A Generalised Instrument. By Sir Robert Ball, F.R.S, 



In the altazimuth the telescope should be perpendicular to a horizontal axis II. 

 which is itself carried by a vertical axis I. Owing to instrumental imperfection the 

 angle between the telescope and the axis I. is 90'' + r and that between I. and II. is 

 QO° — q where r and q are small quantities. 



In the almucantar the telescope is inclined to the vertical axis II. at the angle 

 90° + r, and the vertical axis is inclined to the axis of the Earth I. at the angle 

 i)0°—q, where q is the latitude. In this case r and q are not small quantities. 



Thus an investigation in which the circle attached to axis I. has X for the 

 longitude of its ascending node on the fundamental circle and 6 for the angle 

 between axis I. and the normal to the fundamental circle will include the com- 

 plete theory of the following instruments among others : the Altazimuth, the 

 Meridian Circle, the Prime Vertical Instrument, the Equatorial, and the Almu- 

 cantar. The instrument so defined we call the f/eneralised instrument. 



Let a, S be the co-ordinates of a star and R, Rj be the readings of Circle I. 

 and Circle II. when the telescope of the generalised instrument is directed on 

 that star, then 



SinS=/,(X, ^, y, r .RR,)^ 



Cos a cos S =/,(X, ^, y, ?• . R Ri") ^ (i) 



Sin a cos b-f.^i\, 6, q, r . R Rj) J 



The following is the most convenient method of writing the formulae : — 



Sin 8 = L cos 5 + M sin d "^ 



Cos a cos S = (L sin ^ — M cos 6) sin L + N cos \ |> ? , (ii) 

 Sin a cos_8 = - (L sin ^ - M cos, JS) cos X + N siu X J 



