ON THE SEICHES OF LOCH EARN. 



391 



though it was known that at one time on Loch Treig the back-lash amounted to about 

 1 inch. 



On Loch Earn, where the seiches are normally smaller, and where, moreover, the 

 observations were made at the nodes, damping of the seiches was very noticeable — 

 amounting, in fact, to the total obliteration of seiches of less than half an inch. 



Fig. 21.— A : The Sarasin Limnograph in its original state, showing the outside parts which were removed. B : The same seen 

 from the front, after modification, showing wire passing directly from float to counterpoise over the two pulleys. 



The back-lash was the least serious difficulty ; it could be minimised, to the 

 extent of being negligible, by occasional overhauling and tightening of the Hookes 

 joints. The friction of the vertical rod connected with the float, where it slides 

 through two squared guides in an iron box, was found to be so serious that the whole 

 apparatus had to be modified. It is only possible for the instrument to give passable 

 results under the best possible conditions — when it is rigidly fixed in a permanent 

 building, sheltered from damp, built absolutely vertical, and with the guides smoothed 



