364 PROFESSOR CHRYSTAL AND MR JAMES MURRAY 



the ends. The weight of the upper roller is '79 kg. (if lb.). As my instrument was 

 intended in the first instance to go at one speed only (approximately 1 mm. per minute), 

 the lower or actual driving roller was geared to the clock by means of a fork on the 

 clock arbor and a cross-piece on the axle of the roller. 



The clock itself was an excellent piece of old work, presented to us by Professor 

 Macgregor from the spare apparatus in the Physical Laboratory, and fitted with an 

 escapement by Mr Ritchie, Edinburgh. 



The float was a hollow zinc cylinder about 4 cm. high, having a diameter of about 

 25 cm., and loaded so as to weigh about 2 "27 kgs. (5 lbs.). The weight of the counter- 

 poise was "68 kg. (1^ lb.). It was found during the preliminary tests of the apparatus 

 that a certain load on the float was necessary to secure that the waggon should answer 

 promptly to a slow fall of the water-level. There was some difference of opinion 

 regarding the exact reason for this ; but there is no doubt about the fact. # 



The well used was a cylinder 6 feet long of 13^ inches (34*3 cm.) diameter, with a 

 short access tube of 1^ inch (2 "8 cm.) diameter at the bottom, and a side-door at the 

 top, which gave access to the band in order to make time-marks, and for other 

 purposes. 



The paper used was 9*75 inches (24"8 cm.) broad. In order to avoid blotting and 

 the accumulation of fluff, a kind having a somewhat smoother surface than the Geneva 

 paper was generally used with the stylographs. The ink employed was a mixture of 

 3 parts of Judson's mauve t dye, 4 parts of glycerine, and 12 parts of water. One 

 filling of the pen lasted for several weeks ; and, once fairly started, the pen wrote for 

 several days without attention. All that was required was to clear the point occasion- 

 ally from dirt and fluff gathered from the paper. The style should not project too far ; 

 and its point should be ground as smooth as possible, so as not to cut or plough up the 

 paper. The pen should allow the ink to run freely so that it even blots at first. After 

 a little the blotting ceases, and the pen settles down to draw a strong and fairly thick 

 line. If the line gets very fine, this is an indication that the style is getting clogged ; 

 and the point should then be cleaned. In order that the pen may work well, the 

 limnograph box must not be left open too much ; otherwise the wind drys the slowly 

 delivered ink too quickly, and the style clogs ; or, if the air is moist, the paper gets 

 sodden, and the pen blots and may tear the paper. 



The construction of the apparatus was ably carried out by Mr A. H. Baird of 

 Edinburgh, and worked admirably during the two months that it was under my 

 charge at St Fillans. This was very creditable to the maker, considering that there 

 was very little time for preliminary tests, and none for subsequent modification. 



In fig. 2 the instrument is shown in its box, which is placed within the little 



* The apparent anomaly may have been due to capillary action on the zinc float. In the fixed limnographs of 

 Forel and Plantamour, the float was surrounded by a band of cloth, so as to be constantly wetted by the water. In 

 my instrument as used on Loch Earn, this precaution was not taken. 



+ Red would have been much better for subsequent photographic reproduction of the limnograms. 



