42 THE POPULAR SCIENCE MONTHLY. 



sea- water at the surface for every month, isothermal lines bring drawn 

 at every 10° from 40° to 80° Fahr. By using three colors, and a dif- 

 ferent arrangement of the figures for each season, all the observations 

 of each month are made separately visible on one sheet in the sjDOt 

 where taken. The sheet thus appears to the eye a continued inter- 

 mingling of curves and figures — blue, red, and black — generally open 

 and easily traced where the observations are moderate in number, but 

 an inextricable tangle where frequent. 



As difference of temperature in adjacent portions of the sea indi- 

 cates difference of density, which in turn denotes a mobility of the 

 waters, that is, oceanic currents, these currents are therefore indirectly 

 shown by this series of charts. 



Third, the Track Charts. Such a quantity and variety of infor- 

 mation is crowded into these, that I despair of giving any intelligible 

 idea of them. 



Imagine an artist perched a thousand feet above the center of New 

 York and provided with canvas on which to delineate the city below 

 — to trace in outline every street, house, and tree ; every railway and 

 telegraph line ; all the moving objects, man, horse, and vehicle — what 

 a complicated picture it would make ! Yet this would by no means 

 represent the intricacy of the network on the Track Charts. On 

 them the experience of a large number of all the vessels that sailed 

 the ocean for a period of fifty years is spread before us. Most promi- 

 nent are their jagged courses from port to port ; along these are sym- 

 bols to represent the direction and force of the wind : roman numerals 

 to express the magnetic variation ; arrows and figures to indicate the 

 set and strength of currents ; figures to show the temperature of the 

 sea-water ; great circle routes ; trade-wind limits ; the name of each 

 ship and date of making the passage — and all this in distinctive colors 

 and peculiarity of line, so that each item can be determined with great 

 exactness as regards both time and space. 



Indeed, this profuse interweaving and crossing of lines and figures 

 taxes the patience of even the most painstaking mariner. What the 

 charts show forcibly at a glance, are the great ocean highways, but 

 this chiefly by the multiplicity of tracks through the beaten paths, 

 compared with their sparseness over less frequented routes. 



Fourth, the Storm and Rain Charts. For every five degrees 

 square and each month, they clearly show the relative prevalence (com- 

 pared with the whole period of observation) of the following phenom- 

 ena : gales from eight cardinal points of the compass, calms, fogs, 

 thunder and lightning, and rain (including hail, snow, and sleet). 



The arrangement of these charts is excellent, and they are easily 

 understood. 



Fifth and last of the entire set, the Trade- Wind Charts of the 

 Atlantic Ocean. By a judicious use of colors, figures, and lines, the 

 limits of both trades, of the calm belt between, and of the calm zone 



