MEETINGS OF SOCIETIES. 497 
already recorded. Of these 33 have a period of 2 hours 11 minutes, 
five a period of 2 hours 5 minutes, six a period of 2 hours 17 minutes, 
and 10 a period of 1 hour 12 minutes. The periods of those on the 
Lake of Geneva are 72 minutes and 35 minutes. Of those in Lake 
George which have a period of 2 hours 11 minutes, some are the largest 
yet recorded and others only half or quarter of an inch rise and fall; so 
that there must be something which makes or tends to make the period 
2 hours 11 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 on 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. 
Changes of level, &c., are going on in the earth surface, which, from an 
astronomical point of view, are intensely interesting, because they affect 
the instruments, 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 magnified by 
its comparatively enormous extent as to show themselves on the record- 
ing instruments there. The barograph at Sydney has shown long since 
that thunderstorms come on with a sudden rise of the barometer, which 
at times amounts to a tenth of an inch. If such a change could affect 
one end of the Jake for a few minutes it would be equivalent to putting 
suddenly on to it an inch of water, which would make itselt 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 else- 
where 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. I am here assuming that the storms there are of the same 
character as those which pass over Sydney, but jthey may be smaller 
when passing the Lake, and travel more slowly. Certainly the storm 
which I saw coming down the Lake did not travel with anything like 
such velocity. M. Vaucher, who studied for years the motions of same 
kind, which take place in the Lake of Geneva, considered himself 
justified in saying, “The Lake is disturbed when the barometer is 
unsteady, and because of the varying pressure.” From what I have 
seen so far, the first part of this is true of Lake George, but it is not 
because the barometer is unsteady, but because at such times the wind 
is puffy and variable, and imparts to the water its own peculiarity. Of 
the power of the wind to set the water in motion I have mentioned 
several instances to-night, which I need not repeat, but I may add that 
the large impulses come from the north, because, as it seems to me, the 
wind from that direction acting on the water the whole length of the 
lake has greater power than when blowing from the south over a short 
stretch of water, the gauge being fixed about a mile from the south end. 
But, although the wind is such an obvious cause of the phenomena 
under discussion, I think the barometric changes have some share in it, 
and there are some changes recorded which, so far, I am unable to refer 
to any cause. In the European lakes, in addition to the changes of 
Jevel due to rain and evaporation, there are other changes of level 
