20 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



produces, according to the foregoing principles, a gradual and 

 permanent retardation of the stream, independently of the 

 agency of friction ; and this is accompanied by the produc- 

 tion of heat to an amount equivalent to the lost energy of 

 flow. JPr. British Association. 



REPORT OF THE TIDAL COMMITTEE OF THE BRITISH 



ASSOCIATION. 



In the report of the Tidal Committee of the British Associa- 

 tion, Sir William Thomson stated that the chief object of the 

 originators of the investigation was the determination of 

 long -period tides, and particularly the lunar declinational 

 tide and the solar declinational semi-annual tide. The rea- 

 son for desiring the determination of such tides with great 

 accuracy was that this would give a means of estimating, 

 with absolute certainty, the degree of elastic yielding which 

 the solid earth experienced under the tide-generating influ- 

 ences of sun and moon. It was quite certain that the solid 

 earth did yield to some degree, as it must do so unless it 

 were infinitely rigid. It had long been a favorite assumption 

 of geologists that the earth consisted of a thin shell of solid 

 rock, twenty to fifty miles thick, according to various esti- 

 mates, inclosing an interior filled with melted material lava, 

 metals, etc. This hypothesis was, however, untenable, be- 

 cause, were it true, the solid crust would yield with almost 

 as perfect freedom (on account of its thinness and great area) 

 as if it were perfectly liquid. Thus the boundary of the solid 

 earth would rise and fall under the tide-generating influences 

 so much as to leave no sensible difference to be shown by the 

 water rising and falling relatively to the solid, showing that 

 if the earth, as a whole, had an average degree of rigidity 

 equal to that of glass, the tides would be very much dimin- 

 ished from the magnitude corresponding to a perfectly rigid 

 globe, with water like that of our seas upon it. This consid- 

 eration, he had shown, rendered it probable that the earth had 

 considerably more average rigidity than a globe of glass of 

 the same size. The mathematical calculation showed a some- 

 what startling result, to the effect that a globe of glass of the 

 same size as the earth, if throughout of exactly the same ri- 

 gidity as a small glass globe, would yield, like an India-rubber 

 ball, with remarkable freedom to the tide-generating influ- 



