390 PROFESSOR CHRYSTAL AND MR JAMES MURRAY 



lag ; but there can be no doubt that 1-inch tubing would be ample for that size of 

 well, and for any reasonable length, say up to 100 feet, and probably much more. 

 More accurate experiments under favourable conditions are wanted. 



Experiments made at a later period with smaller sizes of tubing, led into a 6-inch 

 well, showed that a slight decrease of diameter increased the amount of lag, while a 

 considerable increase of length had no perceptible effect (within the narrow limits of 

 our experiments — 20 feet of |--inch tube, for example, caused no visibly greater lag 

 than 10 feet). 



Float and Counterpoise. — Our experience shows that the counterpoise should be 

 much lighter than the float. In the Sarasin limnographs as originally set up, the 

 float and counterpoise were too nearly balanced. (This is no defect in the instruments, 

 as the floats were not supplied with them but made in this country.) In practice the 

 exact ratio of the weight of the float and counterpoise is not of vital importance, and we 

 got satisfactory results with the float three or four times as heavy as the counter- 

 poise. The essential thing is that they be not too nearly equal, and the float should 

 be not less than twice the weight of the counterpoise. 



When they are too nearly alike in weight, the effect of friction at the pulley 

 bearings is increased. In instruments of this sort, in which the movements are 

 extremely slow and gradual, it is inevitable that friction will give rise to slight 

 retardation, and the record therefore proceeds by a series of minute steps. In the 

 traces made by the Sarasin limnographs, before they were modified, the steps were 

 very large. With the direct acting limnograph, and the weight and float bearing the 

 proper ratio to one another, the jerks are so diminished as to be negligible, and are 

 even as a rule quite imperceptible. 



A large buoyant float is not desirable. The more buoyant the float, and the 

 greater its height in proportion to its diameter, the greater is the tendency to wobble, 

 whether from slight currents of air, or from vibrations of the water inside the 

 cylinder, such as might be set up by the impact of the waves on its outside. A 

 broad float, relatively low, and projecting as little as may be above the surface of 

 the water, has been found best ; — indeed, a completely water-logged piece of wood, 

 which would not float at all without the assistance of the counterpoise, has given 

 perfectly satisfactory results ; it has the advantage of being free from capillary dis- 

 turbance, as it is always wetted by the water. 



Modification of Sarasin Limnographs. — From previous experience of the two 

 Sarasin limnographs, when in use on Loch Ness and Loch Treig, it was not expected 

 that they would give perfectly satisfactory work with the small seiches of Loch Earn. 

 It was known that there was a good deal of back-lash in the apparatus for converting 

 the vertical motion of the float into a horizontal motion of the pen. It was suspected 

 that there was friction among the rods and tubes controlling the vertical motion. 

 When previously used the limnographs were set up at the end of lochs, where the 

 seiches are greatest — the damping from those defects being therefore less perceptible, 



