ON THE SEICHES OF LOCH EARN. 373 



principle, and was copied from Dines' self-recording pressure tube anemograph, of 

 whose excellent working we had good experience at St Fillans. 



The instrument can obviously be arranged either to increase or to reduce the range 

 of the seiche. That the pen draws ordinates which are circular arcs instead of straight 

 lines is an objection, but by no means so serious in practice as might be supposed. 



A selection of access tubes is supplied with the instrument, and also a well, into 

 which the whole apparatus can be easily packed. 



The Vertical Band Self-recording Limnograph. — If a rectilinear motion of the 

 recording pen be insisted upon, probably an arrangement of the kind represented 

 in the diagram of fig. 15 would be the best. U and V are two pulleys, of the 

 same or of different diameters, on the same axis. A wire or cord from the float, F, 

 drives U. Round V is wrapped the recording band, to which is fastened a swing- 

 gate recording pen, P. The band is stretched by the counterpoise W, and steadied 

 and guided by two small pulleys, E and Q, pushing it in opposite directions very 

 slightly out of the vertical. 



The whole apparatus could be mounted on the top of a suitable well, which would 

 also serve as a case for carrying it. 



The same principle could also be used for a fixed limnograph for recording seiches 

 on a small time-scale. In that case the recording drum would be replaced by a 

 suitable arrangement for drawing a vertical strip of paper past the recording pen. 



Functions of the Well and Access Tube. 



For a variety of reasons the float of a limnograph is usually placed in a cylindrical 

 Well, to which the water of the lake is admitted by an Access Tube, which may be 

 very short, even a mere hole in the wall of the cylinder, or may be of considerable 

 length. The necessity for this arrangement, and its effect on the limnogram, will be 

 best understood by considering the different natural causes that disturb the level at 

 any given point of the lake surface. Omitting artificial disturbances, such as the 

 passage of boats, etc., we have the following : — 



(1) Volume Denivellations, caused by precipitation or by evaporation. The 

 variation due to these is slow ; and, if it is periodic, the period is usually long. 



(2) Persistent Wind Denivellations, due to the heaping up of the water at one 

 end of a lake when the wind blows steadily towards that end, or upon a shallow 

 shelving shore or in a shallow bay, when the wind blows the water steadily towards 

 the shore. These disturbances vary slowly ; their duration is determined by the 

 prevalence of the wind ; and they are usually neither periodic nor rapidly fluctuating. 



(3) Fluctuating Wind Denivellations* due to the propagation of wave trains on 

 the surface of the lake by the passage of wind squalls, and associated with the rapid 

 variations of wind pressure shown by the self- registering anemograph. 



* These were established by observations with the statolimnograph, and will be discussed in a later paper of 

 this series. 



