374 PROFESSOR OHRYSTAL AND MR JAMES MURRAY 



(4) Swell Denivellations. — After a persistent wind has blown for a time over a 

 certain stretch (" fetch " ) of water, a kind of dynamical equilibrium between wind and 

 water is established ; and the surface becomes covered with more or less regular trains 

 of progressive waves. Owing to reflection at banks, retardation at shores and shallows, 

 and probably also to unsteadiness in the wind, there is an interference of superposed 

 trains, which spoils the regularity of the wave pattern and prevents absolutely regular 

 periodicity in the denivellation at any given point. The effect to the eye is, however, 

 a fairly regular pattern of small progressive waves of apparently constant length, 

 usually diversified by wave maxima at approximately regular intervals. On Loch Earn 

 common values for the periods of the smaller waves, and of the wave maxima, were two 

 seconds and thirty seconds respectively. This wave system persists for some time after 

 the wind falls ; and it is at this stage that the name swell (houle, vagues mortes, Diinung, 

 Todte See) is usually applied to it. 



(5) Seiche Denivellations, which are stationary (standing) oscillations of the whole 

 lake, having definite nodes and periods depending on the configuration of the basin. 



If the object be to observe the seiche denivellation as little confused as possible by 

 other disturbances of the lake level, the best place for a limnograph is on a rock or pier 

 jutting into deep water and sheltered as much as possible from strong prevailing winds. 

 We thus escape that part of the persistent wind denivellation which is due to shallow- 

 ness near the shore, and in a great measure also the fluctuating wind denivellation 

 and the swell. The effects of the two last can be further diminished by making the 

 ratio of the diameter of the access tube to the diameter of the well sufficiently small ; 

 for, if this ratio be small, the tube takes a long time to fill the cylinder, and a maximum 

 of level has been followed by a minimum before the rise has produced its full effect. 

 Thus the narrowing or lengthening of the access tube diminishes the amplitude in the 

 limnogram of disturbances of short duration or of short period, and at the same time 

 causes the maxima or minima registered by the limnograph to lag in time behind the 

 maxima or minima of the actual denivellations of the lake. 



It must be observed that the same applies to seiches of short period. It follows, 

 therefore, that by narrowing or lengthening the access tube, or both, we smooth the 

 limnogram, not only by cutting out wind and swell disturbances, but also by reducing 

 the amplitude of seiches of higher nodality. 



In practice it is very often impossible to find a deep-water site for a limnograph 

 anywhere near the part of the lake, e.g. near a node, where we wish to observe. In 

 this case the difficulty is got over by placing the well in any convenient place and 

 running an access tube out to water of sufficient depth, usually 8 to 10 feet. 



It is easy to calculate the damping and retarding effect of the tube and well. In 

 what follows we shall, for convenience, use C.G.S. units. 



Let a and b be the diameters of the cylindrical well and access tube respectively ; 

 / the length of the tube ; x and y the distances of the surface level in the well and in 

 the lake respectively above any fixed point, say the outer end of the access tube. 



