io8 



SCIENCE. 



[Vol. XIII. No. 314 



that the writer is of the belief that all efforts directed toward the 

 accomplishment of the destruction or dissipation of the tornado- 

 -cloud by any known and practicable means, will fail of realization. 

 Mechanical appliances are wholly incapable of successfully cop- 

 ing with the forces of the tornado, which, unlike the whirlwind and 

 waterspout, arises from the unstable state of the atmosphere in the 

 -cloud region, where the tornadic gyrations commence, and are 

 afterwards propagated downward to the earth's surface. The 

 tornado is controlled by the operation of forces far 'above the 

 Sfround. 



A RIVER-PIRATE. 



There is a little river-pirate in eastern Pennsylvania unsus- 

 pected by its rural neighbors. It is in a quiet, well-watered farm- 

 ing district, where the streams, as a rule, are bent only on minding 

 their own business, and not interfering with their fellows ; and yet 

 one of them is a confirmed pirate, and goes on unhindered in its 

 robbery. 



The pirate is Deer Run, and its victim is the north-east branch 

 of Perkiomen Creek. The head waters of the latter have been 

 captured, and led away from the basin where they were born and 

 passed their youth, and thus diverted to swell the surreptitious vol- 



ume of the intruder. The affair has happened in this way. The 

 country hereabout was in ancient times a surface of faint relief, at a 

 lower stand than now, traversed by idle streams ; but, in conse- 

 quence of elevation to a greater altitude, the streams have revived 

 their lost activities, and set to work to sink their channels and open 

 out their valleys in the process of reducing the land to its proper 

 level again, even with the sea ; for land finds its level, like water, 

 but more time is required before the level is assumed. The streams 

 that drained the country when it was elevated adopted such faint 

 inequalities as they then found for their first settlement, and have 

 since been engaged in perfecting their courses as best they could, 

 cleaning them out, deepening them, and adapting them most ex- 

 actly to the best transportation of land-waste. In the processes of 

 adjustment thus called forth, every stream struggling for its own 

 existence, it sometimes has happened that a stream with steep 

 liead waters has seized drainage area from the flat-lying head 

 waters of an adjacent basin ; because, other things being equal, the 

 waste of the surface is fastest on the steepest slopes, and hence the 

 steeper streams have gnawed more quickly into the land-mass than 

 the flatter ones, and the divide between a pair of contesting 

 streams has consequently been pushed in the direction of the 

 fainter descent. 



The abstract possibility of this process cannot be questioned ; 

 but one might well hesitate before accusing so innocent-looking a 

 stream as Deer Run of such underhand designs. Yet the evidence 

 of its piracy is too direct to be doubted. 



In the first place, the region that the two streams drain has been 



accurately surveyed by the Philadelphia Water Department, and 

 the maps thus secured have been published by the Geological Sur- 

 vey of Pennsylvania. The facts of the case are thus brought 

 clearly before the world, after long remaining in unsurveyed ob- 

 scurity. It is from one sheet of these maps that the accompany- 

 ing figure has been traced, omitting the wooded areas and dwell- ■ 

 ings. The smaller map in the corner indicates the location of the 

 district under discussion in the south-eastern corner of Pennsylvania, 

 north of Philadelphia and west of Trenton. In the next place, it is 

 to be noted that the slope of Deer Run from the divide DD is 

 twice as steep to the north-east as is that of its victim to the south- 

 west. Deer Run descends sixty feet in a mile at its head: the 

 Perkiomen branch descends only thirty feet in the same distance. 

 Again : it appears that the two streams, flowing on the same line 

 but in opposite directions, both follow the same bed of shaly sand- 

 stone in the rock formation (triassic) that underlies the district : 

 there is, therefore, no inequality of structure on the two sides of the 

 divide to determine a difference in the rate of head-water weather- 

 ing. In so short a distance as a mile or two, it cannot be thought 

 that there is any difference in rainfall or other climatic element of 

 significance ; and, if exposure to sunshine be a factor of value in 

 aiding the denudation of a surface by strengthening the diurnal vari- 

 ations of temperature in the soil and increasing the number of win- 

 ter thaws, this advantage would be with the Perkiomen. Leaving 

 this aside, it appears, that, except for difference of slope, the streams 

 are in similar conditions, and any inequality in their action must be 

 referred to the control that the slopes exert. As the control exerted 

 by the slopes is distinctly in favor of Deer Run, we must conclude, 

 that, if a patient observer should take his stand on the higher 

 ground near by, he would certainly see the divide DD, migrating, 

 rather slowly to be sure, to the south-west. After a time the upper- 

 most side-stream of the Perkiomen branch, aa, would be tapped by 

 the insidious operations of the pirate ; and, powerless to withstand 

 the temptations of a more facile descent, it would turn from its 

 parent to join the volume of its captor. In time another side-stream, 

 bb, would be led astray ; and thus Deer Run would extend its ter- 

 ritory at the expense of its more inert neighbor, and the divide 

 would in time be shifted to BB. 



Now, it is noticeable that all tributaries thus acquired by the 

 pirate would enter the head of its main channel in a back-handed 

 manner, like the barbs at the point of»an arrow, indicating by this ab- 

 normal arrangement their early training in accordance with the 

 habits of the Perkiomen family, where they were brought up. But 

 if this process is going on now, we must be persuaded that it has 

 been in operation in earlier times also, and that results of the kind 

 now predicted for the coming ages should already be visible as the 

 product of those gone by. Such is undoubtedly the case. Deer 

 Run bears at its head at least three small side-streams, which still 

 manifest in their directions the clearest indications of Perkiomen 

 habit; and thus it must stand convicted not only of piratical in- 

 tentions for the future, but of piratical practice in the past. 



If the reader should, perchancQ, be seriously inclined to geo- 

 graphic study, he may find many accounts of this kind of interac- 

 tion among rivers in the writings of recent authors. Gilbert has 

 considered examples of the process in our Western Territories ; 

 Lowl and Philippson have pointed out a number of instances among 

 the rivers of Europe ; and Helm has shown how the picturesque 

 little lakes at the head of the Engadine result from the capture of 

 head-water streams by the steep-sloping Maira from the more 

 steady-going Inn. As our intimate acquaintance with the geo- 

 graphic development of our country is furthered by the publication 

 of good topographic maps, we shall undoubtedly find many cases 

 of head-water adjustments. The Atlantic-Mississippi divide, from 

 Pennsylvania to Alabama, should be especially rich in them. 



Yet. if what is one man's food is another man's poison, it maybe 

 that what is one man's crime is another's virtue. It is only in false 

 allegory that we can blame Deer Run for having taken what once 

 belonged to the Perkiomen ; and instead of calling the capture of 

 head waters a piratical act, which at best is but an ad captandum 

 term, it should better be regarded as a sharing of another's burden 

 of labor, and a willing assumption by the more active stream of its 

 fair proportion of the work to be done by the whole river system to 

 which it belongs. Instead of gauging the disposition of streams by 



