NATURAL PHILOSOPHY. 163 



The average of the results is usually obtained by inspection. Divid- 

 ing the intervals between slackwaters into quarters, we give the 

 mean results for ihose periods in a table, and usually place upon the 

 chart arrows indicating the direction or set. and write at the extremity 

 numbers showing the velocity or rate in miles per hour. In case of 

 the observations made in Boston Harbour, the results were so unusually 

 numerous, that the lines of direction were confusing to the eye, and the 

 connection between the results was very difficult to seize. Through 

 the pains taken, the motion of the water was traceable in nearly all its 

 peculiarities, from the entrance through the tortuous passages among 1 

 the islands, alternately narrowing and expanding, to the city wharves. 1 ' 

 On the current chart exhibited by Professor Bache, the direction and 

 force of the currents were represented by lines, the distance between 

 which is inversely as the rate in miles per hour. The reciprocals of 

 the number of miles per hour are here represented by tenths of inches, 

 currents of 0.2, 0.5, 1, 1.5, 2 miles per hour being represented by 

 lines parallel to their directions, and distant 0.5, 0.2, 0.1, 0.75, 0.05 of 

 the inch. The chart was on a scale of one 20,000th. The representa- 

 tion on one of the diagrams corresponded to the flood, and on the other 

 to the ebb, referring to the motions of the current from slackwater to 

 slackwater, and not to the tide or rise and fall of the water. If the 

 current stations were very numerous, the straight lines tangent to the 

 curves of motion of the water (set of the current) would become curves. 

 It is easy for the navigator to seize the relations of the currents he 

 will meet, even by these tangent lines, and to avail himself of the 

 knowledge thus imparted of the direct lateral and eddy currents to 

 avoid danger or to secure advantage. 



WHIRLWINDS PRODUCED BY THE BURNING OF CANE-BRAKES. 



MR. OLMSTED read to the American Association, at New Haven, a 

 paper on whirlwinds produced by the burning of cane-brakes in the 

 South. The canes in Alabama often grow to the height of thirty-five 

 or forty feet. They are cut down, and, after drying for about six 

 weeks, fire is applied to them in several places. As soon as the canes 

 begin to burn, the air that is confined in their cells, and the watery 

 vapor, burst them asunder. They generally explode through several 

 cells at once, and thus are split in one continued line. These explo- 

 sions, in burning a large cane-brake, produce a continued roar, like 

 the discharge of musketry from an immense army. On account of 

 the dry, combustible nature of the cane, when kindled, the fire ad- 

 vances with great rapidity, giving out flames of the deepest red, the 

 intensity and richness of which color are incomparably finer than the 

 flames which arise from the combustion of any other kind of wood. 

 Together with the flame, there ascends a very dense, black smoke, 

 resembling that which arises from burning camphor, or from the chim- 

 neys of gas-works or factories where bituminous coal is used. This 

 smoke also far surpassed, in its dense, deep black color, any thing or- 

 dinarily observed. 



The cane-brake visited by Mr. Olmsted covered a space of twenty- 



