ADDRESS SECTION A, B.A. 1876. 259 



cisterns, instead of being open to the atmosphere, 

 were connected air-tightly by a return-pipe with no 

 water in it, it is probable that the observation 

 might be successfully made : but Siemens's level or 

 some other apparatus on a similarly small scale 

 would probably be preferable to any elaborate 

 method of obtaining the result by aid of very long 

 pipes laid in the ground ; and I have only called 

 your attention to such an ideal method as leading 

 up to the natural phenomenon of tides. 



Tides in an open canal or lake of 12 kilometres 

 length would be of just the amount which we have 

 estimated for the cisterns connected by submerged 

 pipe ; but would be enormously more disturbed by 

 wind and variations of atmospheric pressure. A 

 canal or lake of 240 kilometres length in a proper 

 direction and in a suitable locality would give but 

 10 millimetres rise and fall at each end, an effect 

 which might probably be analysed out of the much 

 greater disturbance produced by wind and dif- 

 ferences of barometric pressure ; but no open liquid 

 level short of the ingens czquor, the ocean, will 

 probably be found so well adapted as it for meas- 



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