272 Mr. F. Galton on Meteorological Statistics [Apr. 24, 



a day's sail from any point of L M by the following device. Erect C 

 perpendicular to L M, and equal 

 to C c ; similarly, erect other per- 

 pendiculars (LI',Mm') from points 

 L, M, &c, of which the day's sail 

 along the route has been laid 

 down in the same way that Cc 

 was, and draw with a free hand a 

 line through V, c', m' ; then the per- 

 pendicular distance X x' from any 

 point X on the line L M to its in- 

 tersection at x' with the line V, c', m* 

 will be the length of a day's sail 

 from X, and it can be transferred 

 with a pair of compasses to find x 

 on the line L, X, M. 



Another application of the principle of isochrones is to draw them at 

 distances of 1, 2, 3, &c. days for ships bound towards, not from, a given 

 port. The first day's isochrone requires no explanation. For that of the 

 second day, a few neighbouring points must be selected on the contour 

 already drawn, and from these such small portions as are needed of other 

 isochrones must be constructed ; the line sketched with a free hand to 

 bound these figures will be the isochrone of two days' journey towards 

 the port, and similarly for the third and subsequent days. The lines of 

 shortest passage from curve to curve join the points whence the sub- 

 sidiary isochrones were drawn with the points where the latter are in 

 contact with their bounding line. The appearance of a chart so con- 

 structed would be that of a series of roughly concentric curves round the 

 port and of radial lines of shortest passage. I give no illustration of 

 such a chart, because it seemed a waste of labour to calculate one upon 

 the scanty data available at the present moment, when far more ample 

 and trustworthy materials have 



been collected and are in the course 

 of gradual publication by the Me- 

 teorological Office; and calcula- 

 tions would have been necessary, 

 because the machine just described 

 exists only in the form of a rude 

 model. 



We have, lastly, to explain how 

 the navigator would use one of these 

 charts on specific occasions. Sup- 

 pose (fig. 5) he finds himself at A 

 with such and such weather, present 

 and probable, how should he steer ? 



Fig. 5. 





V 











1 _ 



















He must use an isochrone appro- 



