Mr. W. Huggius on the Spectra of the Sun §c. 55 



When the velocity of the floating body is less than the speed of 



advance corresponding to the depth to which it disturbs the liquid in 



its immediate neighbourhood, it is probable that the virtual depth of 



disturbance of parts of the liquid bevond the immediate action of the 



s 2 

 floating body adjusts itself to the velocity, and assumes the value -. 



1 1 . Possibility of Obliquely Advancing Tidal Waves. — It is pos- 

 sible that instead of a depth less than the virtual depth corresponding 

 to the speed of advance of a tidal wave, the ridge of that wave may 

 place itself in a position oblique to the parallels of latitude, accord- 

 ing to the principle stated in article 10. It still remains to be ascer- 

 tained, by the study of tidal observations, whether such phenomena 

 take place in the tides of the ocean. 



12. Terminal Velocity of Waves. — It is known that in deep water 

 all waves left free from the action of disturbing forces tend ultimately 

 to assume the condition of free rolling waves, whose velocity of ad- 

 vance depends on their periodic time, and is expressed by the equation 



rp 



a = — . This, then, may be called the terminal velocity of a wave 



27T 



of a given period. It follows that if a wave is raised through the 

 disturbance produced by a solid body, that wave will at first travel 

 with a speed depending on the virtual depth of the original disturb- 

 ance ; but as it advances to a greater and greater distance from the 

 disturbing body, the velocity of advance will gradually approximate 

 to the terminal velocity corresponding to the periodic time, and the 

 virtual depth will continually adjust itself to the changing velocity, 

 and approximate gradually to the equivalent pendulum corresponding 

 to the periodic time. Such is the cause of the forward curvature of 

 the ridges of the obliquely diverging waves which follow a ship*. 



May 14. — Lieut, -General Sabine, President, in the Chair. 



The following communications were read : — 



" Further Observations on the Spectra of the Sun, and of some of 

 the Stars and Nebulae, with an attempt to determine therefrom 

 whether these Bodies are moving towards or from the Earth." By 

 William Huggins, F.R.S. 



§ I. Introduction. 

 The author states that at the time of the publication of the " Obser- 

 vations on the Spectra of the Fixed Stars," made jointly by himself 

 and Dr. W. A. Miller, Treas. R.S., they were fully aware that the 

 direct comparisons of the bright lines of terrestrial substances with 

 the dark lines in the spectra of the stars, which they had accom- 

 plished, were not only of value for the more immediate purpose for 

 which they had been undertaken, namely, to obtain information of 

 the chemical constitution of the investing atmospheres of the stars, 

 but might also possibly serve to reveal something of the motions 



* This is explained in greater detail ia a paper read to the Institution of Naval 

 Architects on the 4th of April 1868. 



