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348 Address of Sir Wm. Thomson at the Glasgow Meeting. 
a space equal to tzssb000 Of the length of the thread. No 
ordinary plummet or spirit-level could give any perceptible 
indication whatever of this effect; and to measure its amount 
it would be necessary to be able to observe angles as small as 
tzo0doc000 Of the radius, or about z4,”. Siemens’ beautiful 
hydrostatical multiplying level may probably supply the means 
smaller or southern cistern will rise from its lowest position to 
its highest position while the moon is rising to maximum altl- 
tude, and fall again after the moon crosses the meridian till she 
sets: and it will rise and fall again through the same range 
from moonset to moonrise. If the earth were perfectly rigid, 
and if the locality were in latitude 45°, the rise and fall would be 
half a millimeter on each side of the mean level ; ora little short 
of half a millimeter if the place is within 10° north or south 
latitude 45°. If the air were so absolutely quiescent during the 
observations as to give no varying differential pressure on the 
two water surfaces to the amount of z$5 millimetre of water, oF 
rzsx of mercury, the observation would be satisfactorily pract 
cable, as it would not be difficult by aid of a microscope 
serve the rise and fall of the water in the smaller cistern 
téo of a millimeter; but no such quiescence of the atmosphere 
could be expected at any time, and it is probable that the vari 
tions of the water-level due to difference of the barometric pres 
sure at the two ends would in all ordinary weather quite ov 
power the small effect of the lunar tide-generating motive. 
however, the two cisterns instead of being open to the = 
phere were connected air-tightly by a return pipe with no vt 
in it, it is probable that the observation might be success “o 
made: but Siemens’ level or some other apparatus sien Pa 
larly small scale would probably be preferable to any elabora 
method of obtaining the result by aid of very long pipes eee 
the ground ; and I have only called your attention to e ere 
Tides in an open canal or lake of twelve kilomete - 
wo of just the amount which we have estima be enor: 
cisterns connected by submerged pipe; but would be 
