ANNIVERSARY ADDRESS. 17 
fifty-four of the best defined amongst those already recorded. Of 
these, thirty-three have a period of two hours eleven minutes, five 
a period of two hours five minutes, six a period of two hours 
seventeen minutes, and ten a period of one hour twelve minutes. 
The periods of those on the Lake of Geneva are seventy-two 
minutes and thirty-five minutes. Of those in Lake George which 
have a period of two hours eleven minutes, some are the largest yet 
recorded, and others only half or a quarter of an inch rise and fall ; 
so that there must be something which makes or tends to make 
the period two hours eleven minutes. It is noteworthy that at 
Lake George as well as the Lake of Geneva the short seich is not 
half the long one, but they bear about the same proportion one 
to the other in each case. 
As to the cause of these motions in the lake I am not prepared to 
say much at present. Further investigation is needed, and I hope, 
by the aid of a recording aneroid already there and a recording 
anemometer to be erected shortly, to be able to compare the changes 
of wind and pressure with the changes in the lake; but I do not 
expect to find everything explained thus. Changes of level, &e., 
are going on in the earth-surface which, from an astronomical point 
of view, are intensely interesting, because they affect the instru- 
ments, and therefore the measures. They are very minute, and we 
have no means of keeping a continuous record of them; but it is 
possible that if such changes affect the lake, they will be so mag- 
nified by its comparatively enormous extent as to show themselves 
on the recording instruments there. The baragraph at Sydney has 
shown long since that thunder-storms come on with a sudden rise 
of the barometer, which at times amounts to a tenth of aninch. If 
such a change could affect one end of the lake for afew minutes it 
would be equivalent to putting suddenly on to it an inch of water, 
which would make itself known at once by a rush to the other end ; 
but although such changes must have some effect, I do not think 
it can be considerable, because, as I have elsewhere shown, these 
storms move at the rate of about 60 miles per hour, and are often 
70 miles wide, so that such a storm coming on to the lake would 
spread all over it too rapidly to cause much motion in the water. 
