ON THE SEICHES OF LOCH EARN. 393 



The limnograph in its altered state was a much more useful instrument, and for the 

 first time began to record seiches of the full amplitude. Further alterations were 

 afterwards made. The travelling rod which carries the pen in the Sarasin limnograph 

 was not so satisfactory after the machine was rendered direct acting. The construction 

 of the metal framework required the driving wire to pass over both pulleys, either of 

 which might at any moment take the lead. The resultant " quarrel " might cause jerks. 

 The defect was perhaps mainly hypothetical, and might have no real existence if the 

 two pulleys were precisely alike in diameter ; nevertheless, it was desirable to remove 

 even hypothetical defects, and a further change was made. 



Professor Chrystal had had a new limnograph made by Baird of Edinburgh, which 

 was also direct acting, and had as a special feature the carrying of the pen by a little 

 waggon which ran by means of three wheels on a pair of rails. 



It was decided to adapt the railway and waggon to the Sarasin limnographs. An 

 improved form of waggon, more convenient to attach and detach, was made by Mr 

 M'Kinstry of Comrie, and proved very successful in use. 



The waggon and clip for holding the pen are shown in fig. 22. 



The Index Limnographs. 



The first two portable limnographs constructed for Professor Chrystal were found to 

 work admirably. They were, however, somewhat inconvenient to set up, and they 

 were too large to be easily portable on bicycles. To obviate these objections, much 

 smaller and lighter instruments were made after my design, which could be carried 

 about without difficulty. The form finally adopted is shown in the accompanying 

 photograph (fig. 23). 



The essential parts of the instrument are the float and counterpoise, connected by a 

 line which passes over a vertical pulley ; a pointer attached to the pulley or to its axle ; 

 a scale on which the movements of the pointer can be read ; a well for the float, and a 

 syphon connecting the well with the lake. The whole apparatus is supported by an 

 ordinary camera tripod. 



The Float. — A circular disc of hard wood, 4 inches in diameter by \ inch thick, was 

 found sufficient. When water-logged, this was very little affected by the wind. Hollow 

 zinc floats were first used, but they were too unsteady if any wind was blowing. 



The Line. — Fine waterproofed fishing-line was used. We were unable to ascertain 



whether the line stretched and contracted much with varying degrees of moisture and 



temperature. In the course of an observation it was commonly the case that the mean 



level steadily rose or fell ; more rarely it would rise for a time, then fall again, or vice 



versd. These changes could be caused by changes in the length of the line, or they 



might be real changes in the level of the loch. When the index was used close to a 



permanent limnograph, both instruments usually indicated the same changes of level, 



showing that these were really occurring in the loch. 



TRANS. ROY. SOC. EDIN., VOL. XLV. PART II. (NO. 14). 55 



