ON THE SEICHES OF LOCH EARN. 389 



The Limnograph. — A detailed description of the Sarasin limnograph is not called 

 for here. Its essential parts will be understood from the photograph reproduced in 

 fig. 2. The modifications that were found necessary during our experiments will be 

 explained in due course. 



The Cylinder. — All the limnographs belonging to the Lake Survey were fitted with 

 cylinders of 13^-inch inside diameter. This is probably somewhat excessive, and 

 cylinders of 1 2 inches should be ample. The water is admitted to the cylinder by a 

 circular opening, in which is fixed a short piece of brass tube, to which the long tube 

 leading to the deep water can be attached. It is convenient to have this short piece 

 of such a diameter that two standard sizes of rubber tubing can be easily and quickly 

 connected with it. If it is made of 1-inch inside diameter, a f-inch rubber tube can 

 be slipped inside it, or a 1-inch tube outside. Other sizes can be attached by using 

 short connecting pieces of various diameters. It is needless, and in stormy weather 

 objectionable, to have brass couplings which require to be screwed on. 



The Long Tube. — The tube leading from the cylinder to the moderately deep water 

 of the loch is an important part of the apparatus. It is designed to eliminate the effect 

 of short waves. Lead tubing was first used ; it was found difficult to lay this without 

 bays in which air might accumulate, and it was discarded in favour of rubber hose-pipe. 

 The length of pipe necessary varies with the local conditions. In Loch Earn, a length 

 of 60 feet was sufficient to reach a depth of 10 feet or upwards, which was enough to 

 purify the curve of disturbances peculiar to shallow water. The outer end of the tube, 

 lying on the bottom of the loch, is apt to get choked with mud. This is prevented by 

 passing the end of the tube through perforations in a cubical box, which in any position 

 will keep the tube 1 foot or so off the bottom. 



The ratio which the length and diameter of this tube should bear to the diameter of 

 the cylinder is very important, and Professor Chrystal made a number of calculations 

 of what it ought to be in certain cases. These need not be entered into here, as we are 

 only concerned with the practical manipulation of the apparatus. Following on 

 Professor Chrystal's calculations, a number of experiments were made with different 

 sizes of tubes. 



The aperture in the cylinder was plugged and a measured quantity of water intro- 

 duced. Then, the plug being removed, the rate of leak through tubes of various 

 diameter was timed. Defects of the Sarasin limnograph unfortunately rendered this 

 experiment worthless. 



We then attached 60 feet of f-inch lead tubing to the cylinder, and compared the 

 seiche tracing obtained through this with one read simultaneously from an index 

 limnograph. We thought we detected lag, and there was certainly damping of the 

 seiche. We therefore adopted for both limnographs tubing of 1^-inch diameter. It 

 was afterwards ascertained that the damping was due to back-lash and friction in the 

 instrument. It was also found that when the f-inch tube was led into a 6-inch well, 

 there was no perceptible lag. The f-inch tube with a 13|--inch well might cause some 



