200 Mr. E. Roberts. On a new Tide-Predicter. [June 19, 



period of the chief component (the mean lunar semidiurnal) relatively 

 to the mean solar semidiurnal is inappreciable during a whole year's 

 predictions, amounting to about o, 10 only in a period of fifty years. 

 The largest deviation from strict accuracy is 0°'37, after a run repre- 

 senting twelve months. This is, however, of one of the very small 

 components, and insensible in its results. This part of the design may 

 be therefore regarded as practically perfect. 



Each component is provided for setting with a crank, in which a 

 sliding piece is fitted, carrying a steel guiding-pin. The guiding-pin 

 is thrown out by means of a fine-cut screw and micrometer head. An 

 improved parallel slide, carrying a pulley, is also' fitted to each compo- 

 nent. The guiding-pin works between two parallel adjustable steel 

 jaws at the back of the pulley frame. The pulley frame is fitted with 

 a balance- weight, so that its centre of gravity is in a vertical line 

 through the pulley's axis. The whole slide is counterpoised by a cord 

 and weight, passing over pulleys, in order to relieve the guiding-pin of 

 all strain and to prevent wear. The steel bar of the pulley slide moves 

 freely in two guides drilled out nearly their entire length to reduce 

 the touching parts to a minimum. The other side of the pulley slide 

 is kept in position by a projecting fork or guide, travelling with free- 

 dom along a narrow flat brass bar. Both the brass bar guide and the 

 steel rod guide are divided to millimetres ; the brass bar for approxi- 

 mate, and the steel rod for the accurate, adjustment of the throw of 

 the crank-pin, for which purpose the upper guide of the steel rod is 

 furnished with a vernier. The milled head of the micrometer- screw is 

 also divided and may be used with the divisions on the brass bar guide. 

 The pulley frame is movable on its steel rod, for the purposes of the 

 perfect adjustment of the pulley about the centre of motion of the axis 

 of the crank. 



The axis of the crank carries behind the main plate a fine-toothed 

 wheel, fitted on a slotted cone, with a milled nut for clamping the 

 wheel on its axis. The toothed wheel is driven by an endless screw, 

 carrying a bevelled wheel, which is itself driven by another bevelled 

 wheel on one of the four main axes of the machine. The endless 

 screws and main axes are fitted with counter pivots. 



At the back of the machine are fitted the setting dials. Each dial 

 is toothed round its outer edge and movable round its centre by a 

 pinion for adjustment. The axis of the component projects through 

 the setting dial, and carries a steel pointer for setting. 



A fine flexible wire fixed to a large screw-head passes alternately 

 under and over the pulleys of the lower and upper series of com- 

 ponents, and carries an ink-bottle at its free end. The iuk-botile, 

 fitted with a fine glass point, travels in a geometrical slide, and is 

 suspended to give just sufficient pressure to ensure contact on the 

 paper of the recording barrel. 



