138 Mr Newall, On a combination [Jan. 29, 



rp,, dy 2n7r b . 2w7r 



inen -~- = — ^ sm — --a;. 



ax a\ 2, a\ 



Hence — - will represent the maximum tilting, a being the 



earth's radius, and b the range of the wave from highest to lowest. 

 For illustration suppose the latitude to be 60°, and the age of the 

 tide two days, and the range of the wave six feet. This would 

 indicate a tilt of 0"35 of a second, which would be noticeable with 

 Mr H. Darwin's pendulum. 



Attention has lately been given to small disturbances of the 

 surface, and it was only last week that a short article on " earth 

 movements " by Prof, Milne appeared in Nature, in which he 

 tells us of a " tide-like movement of the surface of the earth." 

 He adds that the effect is " too large for a terrain tide produced 

 by lunar attraction." This can only mean that the deformation 

 which has been observed is larger than the deformation which 

 lunar attraction would produce in the earth : because any de- 

 formation directly produced by lunar attraction would not be 

 indicated by any effect on Mr Darwin's pendulum, unless the earth 

 was rigid. But disturbances of the surface by deflected tides in the 

 substratum in the manner now suggested would be noticeable by 

 means of that pendulum, and they would have a greater rise and 

 fall in comparison with a " terrain tide " caused by lunar attraction, 

 just as the tides on our shores rise and fall much more than would 

 happen in the case of a rigid earth wholly covered with water. 

 In the case of deflected tides as suggested, the tilt would in 

 general be in some direction differing from east and west, always 

 the same at the same locality, but altering from one locality to 

 another. All these points are for observation. The disturbances 

 would need to be disentangled from those arising from barometric 

 changes, and their periodicity made out. If caused in the manner 

 suggested, they ought to alternate every six lunar hours, and to be 

 most marked at spring tides. 



(5) On a combination of prisms for a stellar spectroscope. 

 By H. F. Newall, M.A., Trinity College. 



The arrangement of prisms, which is the subject of this note, 

 has not so far as I am aware been described before, and its con- 

 veniences, for astronomical purposes in particular, are so numerous 

 that I propose to give some details concerning it. 



ABC is a strongly dispersing prism ; all three faces are accu- 

 rately worked, and the angles at A and B are equal. 



DEF is an ordinary double total reflexion prism. 



