258 



NATURE 



[July 29, 1875 



weapons ; tents and boats of various kinds, special in- 

 struments and apparatus for polar expeditions, &c., not 

 to mention narratives and publications of every kind 

 relating to voyages. 



How varied the programme of this exhibition is will be 

 seen from the above, as well as the fact that the geo- 

 graphy of the present day is a very complicated and all- 

 embracing province of knowledge indeed. 



It is impossible here to analyse in detail the exhibits 

 of each country ; we can only at present refer to some of 

 the objects which, as we learn from a correspondent, have 

 attracted considerable attention. 



The fine set of instruments for travellers exhibited by 

 our Royal Geographical Society, and invented by Capt. 

 Georges, R.N., seems to have excited considerable atten- 

 tion ; it includes a double pocket sextant, an artificial 

 horizon, and a barometer ; the latter especially, on 

 account of its ingenious construction, making it useful in 

 mountaineering, is said to have attracted the attention of 

 the New French Alpine Club. 



From Norway comes a very simple declimeter hav- 

 ing a crank working on a small notched wheel which 

 multiplies by ten the number of degrees on the limb on 

 which the readings are taken ; a close approximation can 

 thus be obtained by a very simple contrivance. 



A Russian marine officer has sent a compass magnifi- 

 cently fitted up, and a lead for taking soundings, and 

 samples of the bottom in lakes and shallow seas. It 

 was used with success on Ladoga, the Caspian, and the 

 Baltic. The apparatus is very simple, cheap, and not 

 ponderous. 



Mohn's map of churches struck by lightning in Norway 

 is exhibited in order to illustrate the special danger of 

 lightning to churches. It shows that two churches in 

 every three years are struck and one of the two is utterly 

 destroyed, and that in a climate where thunderstorms are 

 relatively infrequent. 



Sweden exhibits two wonderful pieces of apparatus. 

 The first is a meteorograph for printing in numbers the 

 degrees of dry and wet bulb thermometer, barometer, and 

 the force of the wind. The types are placed on wheels 

 which are moved every quarter of an hour by electricity. 

 The barometer is a syphon one, and the thermometers 

 open by the top a needle which descends every quarter of 

 an hour into the mercury and gives the degree. The 

 apparatus works regularly at the University of Upsal 

 and at the Vienna Observatory, where the readings have 

 been found quite correct. The printed sheets obtained at 

 Upsal are posted on the wall of the Geographical Ex- 

 hibition. 



A Swedish engineer has invented a machine to show 

 where to find beds of iron ore, and to determine also the 

 depth to which it is necessary to descend. The miracle is 

 performed by tracing on a map isodynamic magnetic 

 curves, with a compass exposed to the perturbating influ- 

 ence of a magnetic needle placed at a distance. Two 

 systems of isodynamic curves are to be traced, and the 

 distance between both centres is proved to indicate the 

 depth. Experiments and explorations with this extraordi- 

 nary instrument have proved successful. 



The Belgian universal meteorograph, as used in Ghent, 

 is said to be the great success of the Exhibition. It is 

 expected to create a revolution in weather-warnings and 

 in meteorology generally, and will leave the famous 

 Greenwich registering apparatus far behind. A reading 

 is taken every quarter of an hour and engraved on copper 

 ready for going through the press. The inventor is M. Van 

 Ry sselberghe. Professor to the N avigation S chool of O stend. 

 The members of the several juries visited the galleries 

 of the Exhibition on Monday last for the first time. Many 

 members of the Academy of Science — MM. Leverrier, 

 Faye, Quatrefages, and others — were present, as well as 

 the foreign commissioners. We hope to give further 

 details next week. 



THE REGULATION OF RIVERS 



nr HE recent disastrous floods in France and England 

 -»■ call attention to the question whether it is practi- 

 cable so to regulate the flow of the water in rivers as to 

 prevent, or at least greatly diminish, such misfortunes for 

 the future. Facts and numerical data exist which show 

 that such regulation is practicable with much less difficulty 

 and cost than would be thought by any one who had not 

 made the necessary calculations. 



It is perhaps scarcely necessary to say that the method 

 of keeping the floods off the lands by means of embank- 

 ments, which is the only possible resource when we have 

 to contend against the sea or tidal rivers, is totally inappli- 

 cable to the case of the inundations of mountain streams 

 like the Garonne. There need not be any difficulty as 

 to the strength of the embankments, but it would be im- 

 practicable to make them high enough to contain between 

 them such torrents as that of the Garonne when in flood. 

 The only way in which mountain torrents can be regu- 

 lated is by constructing reservoirs to retain the flood- 

 water : and the more this plan is looked at, the more 

 feasible it will appear. 



We shall first refer to a paper by Charles EUett, jun., 

 C.E., on " the Physical Geography of the Mississippi 

 Valley, with suggestions for the improvement of the navi- 

 gation of the Ohio and other Rivers," forming part of the 

 " Smithsonian Contributions to knowledge " for 185 1, pub- 

 lished by the Smithsonian Institution, Washington. 



This paper contains the tabulated results of an elaborate 

 series of observations made by the author in the spring 

 and summer of 1849 on the flow of the Ohio, at Wheeling, 

 between Pittsburg and Cincinnati. The flow varied from 

 10,158,000 cubic feet per hour, with a depth of 2.20 

 feet on the bar at Wheeling, to 736,000,000 cubic feet, 

 with a depth of 31 '25 feet on the bar. 



" The average volume of water annually flowing down 

 the Ohio is 835,000,000,000 (eight hundred and thirty-five 

 thousand million) cubic feet. This volume would fill a 

 lake 100 feet deep and 17J miles square. To have regu- 

 lated the supply of the river in 1848, so as to have kept 

 the depth on the bar at Wheeling uniform throughout the 

 year, would have required reservoirs capable of holding 

 240,000,000,000 cubic feet, which is equivalent to a single 

 lake 100 feet deep, and 9 J miles square. There is no 

 difficulty, on any of the principal tributaries of the upper 

 Ohio, in obtaining reservoirs capable of holding from 

 twelve to twenty thousand millions of cubic feet. It can 

 scarcely be doubted that twelve or fifteen sites for dams 

 may be selected capacious enough to hold all the excess 

 of water, and equalise the annual discharge so nearly that 

 the depth may be kept within a very few feet of an in- 

 variable height." 



To control the floods of the river, however, much less 

 than this would be needed. Mr. Ellett takes the case of 

 the flood of March 1841, as being that in which the 

 greatest quantity of water passed down of all the floods 

 concerning which he has information. He takes 25 feet 

 of depth on the bar as the high-water mark, above which 

 the river is in flood ; he estimates that during nine days 

 of flood the river passed down 159,000,000,000 cubic feet 

 of water, while during the same time, had it been steady 

 at the high-water mark, the discharge would have been 

 only 115,000,000,000. If consequently the excess of 

 44,000,000,000 had been kept back in reservoirs, the flood 

 would have been prevented. 



The volume it is here proposed to deal with — 

 44,000,000,000 cubic feet — is "just equal to the quan- 

 tity the river would discharge in fifty days when there is 

 a depth of five feet in the channel." 



The valley of the upper Alleghany, one of the [tribu- 

 taries of the Ohio, is about a third of a mile in width, A 

 dam from 55 to 60 feet in height, thrown across the 

 trough of this valley, so as to submerge not only the main 



