1873.] in determining the best Course for a Ship. 271 



quarter whose initial is opposite to the zinc plate marked 0, under the 

 several influences of the N., E., S., and ~W. winds as they are found to 

 occur. What remains is to sum up these movements. 



I put a pulley, running easily on its axis, upon each of the upper slides, 

 as in the figure, and pass a band, one of whose ends is secured to a fixed 

 peg, round these pulleys, alternately over and under them. The free end 

 of the band is kept stretched by a fight weight, and a framework, carry- 

 ing a vertical pricker, is urged to and fro by the movements of the band. 

 A disk (D), upon which the drawing-paper is secured, has its centre 

 exactly below the pricker when the machine is at zero ; and whenever A 

 is pushed home, the pricker travels in a radial distance to an amount 

 equal to twice the sum of the movements of the several slides, and there- 

 fore through a distance proportionate to a day's sail of the ship towards 

 the quarter whose initial letter is opposite to the zinc plate marked 0. If 

 desired, the numerical value of the movement of the string could of course 

 be read off. 



The manipulation of the instrument would be as follows : — 

 1. Eemove A A. 2. Push or pull the slides n, e, s, w into their mean 

 position. 3. Thrust a skewer, S S, through the holes in the arms and 

 framework to hold everything fast. 4. Adjust the centres, and clamp 

 them if necessary. 5. Adjust the bars for force of wind. 6. Eemove 

 the skewer. 7. Push the slides n, e, s, iu as far back as they will go. 

 8. Eeplace A A, and pull the slides forward to it. The machine is now 

 in working order, a. Push A A home. b. Press the pricker, not for- 

 getting to mark the N point, c. Turn the disk through a quadrant. 



d. Pull back A A, and set it one step in advance on the tramways. 



e. Pull the slides up to it. 



The series 1 to 8 has to be gone through once for all for each isochrone ; 

 that from a to e for each of the 4 points of the compass. The whole of 

 these actions are simple and rapid, and the adjustments are of the easiest 

 kind. An isochrone based upon 4 points ought to be leisurely plotted out 

 in 2| minutes, and one based upon 8 points in 4 min utes. 



This step-by-step arrangement is far easier of construction than one, 

 which may suggest itself to many persons, in which the movement should 

 be continuous, using a curved surface instead of a set of curved edges, and 

 by which the entire curve should be drawn instead of a few points 

 pricked out ; also it is far more convenient and compact not to arrange 

 the machine in a circular form, which is that which would most naturally 

 first be thought of. 



When the isochrones have been drawn to scale on a chart, isochronic 

 lines at various points along any proposed route could readily be found by 

 graphical interpolation. Thus, in fig. 4, let the route be from L to M, 

 and let the isochrones round A and B be known. Draw Aa,Bb parallel 

 to L M, and join A B, a b, cutting L M in C and c ; then C c is one day's 

 sail from C. We can do more than this ; for we may find the distance of 



