ON THE SEICHES OF LOCH EARN. 363 



wheels be well centred and run easily ; also, the waggon should have sufficient weight, 

 and be adjusted at a proper height to prevent tendency in any of the wheels to rise 

 from the rail, owing to the pull of the band. The recorder used was one of the shorter 

 varieties of the common stylographic pen. The pen was carried by a split tube, hinged 

 on a horizontal axis pivoted in two conical bearings, one of which was mounted on a 

 spring of sufficient strength to take up any looseness and prevent all possibility of 

 shake. Usually the weight of the pen itself tilting round the axis gave sufficient 

 pressure on the paper ; but this pressure could be adjusted by pushing the pen 

 backwards and forwards through the split tube, or, if necessary, by using a counter- 

 poise. There are, of course, limits to the angle at which the pen writes satisfactorily. 

 Between the two pulleys and close to them are two vertical plates, each having a small 

 slot through which the band passes ; these limit the motion of the waggon, so that 

 the pen cannot pass off the paper. The waggon and pen are shifted backwards and 

 forwards, as the mean level of the lake requires, by means of the screw clamp ; and the 

 graduations on the Chesterman's tape enable us to keep a record of the height of the 

 water by merely reading off the graduations at the edge of the clamp. 



The rest of the apparatus closely follows the Sarasin instrument. There is a 

 removable stock-drum for holding the roll of unused paper, on which I found it an 

 improvement to put a small friction brake ; this gives a more uniform tension to the 

 paper, and steadies the clock by giving it more uniform work to do — as the varia- 

 tion of the pull, owing to the variation of the diameter of the roll of paper, can be made 

 a small fraction of the pull due to the brake. From the stock-drum the paper passes 

 over a guide-drum, then over a horizontal writing-table, then between the two draw- 

 rollers, the lower of which is driven by the clock ; and is finally delivered freely into 

 the bottom of the box which contains the limnograph. A store roller is added on the 

 delivery side, on which the paper delivered is rolled up from time to time, and kept 

 until it is convenient to remove it from the instrument. 



As in Sarasin's limnograph, a time-marker is fixed at the end of the writing-table. 

 This consists of a stylograph mounted exactly like the recording pen, except that it is 

 attached to a plate which can rotate about a vertical pin. This plate when at rest is 

 kept in contact with a fixed stud by a tension spring ; but once every hour a cam 

 carried by the arbor of the driving clock causes the plate to jig a little round its axle- 

 pin, so that the straight line drawn by the time-pen shows a small V-shaped 

 indenture. These indentures are useful for counting hours; but for accurate time we 

 relied on a series of time-marks made once or twice a day or oftener by giving the band 

 of the limnograph a slight pull, so that the recording pen made a line across the limno- 

 gram. Opposite this line the date and time of day were marked on the paper. No 

 attempt was made to regulate the driving clock, which was left alone as much as 

 possible, the only demand made upon it being uniformity of driving. 



The driving rollers were made of solid gun-metal ; they were slightly hollowed 

 out in the middle, and grip the paper merely between slightly roughened portions at 



