Address of Sir Wm. Thomson at the Glasgow Meeting. 849 
mously more disturbed by wind and variations of atmospheric 
ressure. A canal or lake of 240 kilometers length, in a proper 
direction in a suitable locality, would give but ten millimeters 
rise and fall at each end, an effect which might probably be ana- 
lyzed out of the much greater disturbance produced by wind 
and differences of barometric pressure ; but no open liquid level 
short of the ingens cequor, the ocean, will probably be found so 
well adapted as it for measuring the absolute value of the dis- 
turbance produced on terrestrial gravity by the lunar and solar 
tide-generating motive. But observations of the diurnal and 
semi-diurnal tides in the ocean, do not (as they would on smaller 
and quicker levels) suffice for this purpose, because their 
amounts differ enormously from the equilibrium values on ac- 
count of the smallness of their periods in comparison with the 
periods of any of the grave enough modes of free vibration of 
the ocean as a whole. On the other hand the lunar fortnightly 
declinational and the lunar monthly elliptic and the solar semi- 
annual and annual elliptic tides have their periods so long that 
their amounts must certainly be very approximately equal to 
the equilibrium values. 
ut there are large annual and semi-annual changes of sea- 
level, probably both differential on account of wind and differ- 
ences of barometric pressure and differences of temperature of 
the water, and absolute depending on rain-fall and the melting 
away of snow and return evaporation, which altogether swamp 
the small semi-annual and annual tides due to the sun’s attrac- 
hon. Happily, however, for our object there is no meteorologi- 
Not succeeded in obtaining any trustworthy indications of 
either of these tides. The St George’s pier landing-stage pon- 
— was unhappily chosen, for the Liverpool tide gauge cannot 
: calculation were expended before the long-period tides for 
elbre Island could be attacked, and three years of Kurrachee 
More Pp 
The absence from all the results of auy indication of a 18°6 
