4 6 



from this new standpoint, can be given numerically. Each 

 star has a stream-velocity of about 26 kilometres per second, 

 directed towards R.A. 5h. 44m., Dec. 24, or towards the 

 opposite point according as it belongs to Stream I. or Stream II. ; 

 in addition it has its own peculiar motion, equally likely 

 to be in any direction, and averaging about 28 kilometres 

 per second."* 



The above discussions have been concerned with stars 

 of large proper motions, that is to say, with stars that are 

 relatively near to us. But the inquiry has recently been ex- 

 tended to the proper motions of the Greenwich Catalogue 

 for IQIO,* alluded to above, and seems to indicate that the 

 two great star-streams extend to the nearer borders of the 

 Milky Way. 



Two small telescopes still remain to be noticed, the first 

 of which is devoted to a problem which might almost be de- 

 scribed as the inverse of those just noticed, dealing with the 

 dimensions of the universe of stars and the currents in the 

 great stellar ocean. The Cookson Floating Zenith Telescope 

 does not belong to Greenwich, but has been lent by the Obser- 

 vatory of Cambridge University for seven years, now extended 

 to fourteen. It was devised by the late BRYAN COOKSON 

 (1874-1909), and carries an object-glass of 6-| inches aperture 

 and 65-4 inches focal length, one of Dennis Taylor's triple 

 combinations. The telescope rotates round a vertical axis, 

 the position of which is assured by the support of the tele- 

 scope being made to float in a ring-trough containing mercury. 

 The instrument, which was made by the Cambridge Scientific 

 Instrument Company, has been devoted to the determination 

 of the latitude of the Observatory, and at the date of the 1914 

 Report, 387 latitude plates had been obtained. Just as the 

 apparent place of a star is not invariable, but suffers change 

 from many causes, so the apparent latitude of an observatory 

 suffers change, the axis of rotation of the earth appearing to 

 move round its mean position at a distance from it of about 

 60 feet. Very accurate determinations of the latitude of 

 the Observatory are derived from photographs, taken with 

 the Cookson telescope, of pairs of stars, equal in zenith distance, 

 and the apparent change of a very few feet in the place of 

 the North Pole of the earth is indicated by observations 

 made at a distance from that Pole of 2,700 miles. At the 



* "The systematic Motions of the Stars between Dec. 24 and 42" 

 by Sir F. W. Dyson and Mr. W. G. Thackeray. Monthly notices of 

 the R A-S. for 1917, June; Vol. Ixxvii., p. 581. 



