II. LENGTH. 



69 



effected in the opposite direction by a weight D turning the disc E, connected 

 with the disc C upon the same axis. In order to reduce the movement of 

 the buoy A in a certain proportion, a pinion F, on the axis of the disc C 

 and E, moves a rack G, in a horizontal direction. On one end of G a 

 diamond-point H is fixed, on the other end two rollers J, J, with a horizontal 

 axis. 



Fig. 1. 



The clock of the instrument turns the cylinder round its axis in 24 hours by 

 means of a combination of wheels, and the glass disc M in six hours. 



The tide-curves are engraved upon the cylinder L with the diamond-point 

 II. The rollers J, J move en the glass disc M by the combined action of the 

 buoy A, the weight D, and the clockwork. 



All the different parts of the apparatus are fixed upon the same plate N, N 

 of cast iron, planed at the necessary points to insure their invariable position. 

 The plate N rests on three columns of cast iron, placed upon the coping of 

 the shaft. 



The cylinder is covered with blackened chalk paper, whereon the diamond 

 engraves the tide-curves as white fine lines on a black ground. 



The paper on the cylinder L is divided in half hours, and from meter to 

 meter of height. For this purpose a proper self-acting apparatus is con- 

 structed, by which both divisions are made with great exactness. Diamond- 

 points are used for this purpose also. It is necessary, for the sake of distinct- 

 ness, to renew the covering paper of the cylinder once a month. To avoid 

 waste of time a second exactly similar cylinder is prepared and carefully 

 divided, to replace the former cylinder. 



For the observation of the constants of the apparatus, it is necessary to 

 note complete revolutions of the disc C and glass disc M. For this purpose 

 two indices are applied. 



The circumference of the disc C, measured on the axis of the copper wir 

 B, is exactly two meters in length. 



