620 



SCIENCE. 



[Vol.. II., No. 40. 



Zoom in North Brabant, exported in 1881 about two 

 million oysters, valued at some three million francs. 



In order that the building may be easily taken to • 

 pieces and put in position, it is made entirely of wood ; 

 and the parts are arranged with such care that its 

 removal from one place to another requires only three 

 days besides the transit. 



The station was at Bergen-op-Zoom when we visited 

 it, and we were received by Professor Hubrecht with 

 the cordiality and kindness characteristic of the sa- 

 vants of Holland. It is composed of a principal build- 

 ing about eight metres long and five broad. One 

 facade has four windows; the other, three. The 

 walls are three metres high ; the ridge of the roof, four 

 and a half. The framing of the roof is of wood, cov- 

 ered with a double layer of rush-matting. Opposite 

 each window there is a stationary table : tables are also 

 arranged in the centre of the room. In the laboratory 

 are a closet for the instruments, another for the re- 

 agents and bottles, and also a small library, containing 

 periodicals and the principal works on marine faunas. 

 Each investigator can, in addition, send for books 

 which he needs, either from the library of the zoo- 

 logical society, or from one of the universities of 

 Holland. A desk, foot-rests, and some folding iron 

 chairs complete tlie furnishings. The work-room, 

 properly so called, is entered through a room in which 

 are the aquaria, the collecting-apparatus, and the 

 smaller dredges. The cumbersome instruments are 

 placed in a room connected with one of the side- 

 facades. Another room, opposite the entrance, leads 

 into the private oiBce of the dii'ector of the station. 

 A fence of galvanized zinc wire runs around the build- 

 ing, and, while it wards off the thoughtless, encloses 

 a space which may be used either for experiments in 

 the open air, or for the dissection of animals of large 

 size. 



The construction of "the house, as it stands to-day, 

 has cost fifty-five hundred francs. An additional 

 sum of six thousand francs was expended in the pur- 

 chase of furnishings, aquaria, collecting-apparatus, 

 reagents, thermometers, lenses, etc. 



The management of the station is regulated in a 

 very simple manner. The members of the zoological 

 society nominate each year a committee, which pub- 

 lishes at the end of the year a report of the work, and 

 gives an account of the funds expended. 



Although the resources of the zoological commis- 

 sion are very limited, nevertheless the members have 

 undertaken important work. During the season at 

 Helder, Mr. Hubrecht was engaged upon fishes; Mr. 

 Hoek studied the crustaceans; Mr. Horst, annelids; 

 while Messrs. Van Harem, Noman, and Sluiter stud- 

 ied the other invertebrates. Mr. Hoek undertook, 

 at Bergen-op-Zoom, liis interesting researches on the 

 embryology of the edible oyster. H. E. Sauvage. 



LETTERS TO THE EDITOR. 

 The formation of tornadoes. 



In the discussion of Jlr. Hoy's paper before the 

 American association for the advancement of science, 

 at the recent meeting at Minneapolis, I notice a 



number of .statements which seem to me erroneous. 

 Professor Rowland, for instance, asserts that " the 

 rotation of the tornado is a necessary consequence of 

 the earth's rotation." Now, if this be true, wlgr are 

 not tornadoes more fi'equenf.' Why is it necessary 

 to have brisk, southerly winds, with high tempera- 

 ture and low barometric pressure'? Why do they not 

 happen on clear days as well as on cloudy ones? 

 Again : if the earth's rotation determines the dinec- 

 tion of the gyratory motion of tornadoes, why does it 

 not govern the motion of the little whirlwinds 

 occurring in dry weather? Every observer knows 

 that these revolve, sometimes in one direction, and 

 sometimes in the otlier, but perhaps, in a majority of 

 cases, in the same direction as tornadoes. 



It is well known that tornadoes in our latitude 

 occur on days when there is a strong breeze from a 

 southerly direction. Now, the air on such dajis, in 

 spring and early summer, is heavily charged with 

 moisture; to which fact is due the oppressiveness of 

 the heat. As the heat of the day increases, local 

 showers are formed, which move, not with the surface- 

 wind, but in a higher current from a westerly direc- 

 tion. There is usually a divergence of about ninety 

 degrees in the angle formed by a line indicating the 

 direction of the track of the tornado, and another 

 marking the direction of the preceding surface-wind. 

 Now, the mingling of these currents, or even the 

 passage of one beneath the other, must, on account 

 of their unequal temperatures, condense more or less 

 of the moisture of the warmer current. This con- 

 densation is nearly always noticed, A cloud, often 

 of intense blackness, accumulates just under the 

 southern edge of the storm-cloud, and is usually pro- 

 longed horizontally to the northward along its base. 

 If tlie cloud is near enough, so you can see beneath it, 

 the parts farther in the rear will be seen to move 

 rapidly in an. easterly direction, just as the air-cur- 

 rent moves which bears them. How can it be other- 

 wise than that a gyratory motion shall result from 

 such conditions? People frequently say, in describ- 

 ing a tornado, that " two dark clouds rushed together 

 from opposite directions, and produced the tornado." 

 This statement, if- we are expected to construe It 

 liteially, seems somewhat absurd, since a cloud is 

 always a passive element, moving only as the air 

 moves in which it floals. That fragments of clouds 

 and often quite large masses move toward each other 

 is true; but tornadoes are not produced because of 

 this, but on account of the riiutual resistance of two 

 currents of air. If two liquids or gases are brought 

 together from different directions, a whirling motion 

 is prpduced, as may be seen where two small streams 

 of water flow together, or where a rock or fallen tree 

 interrupts the direct movement of the water. This 

 is but a necessary result of the combination of prop- 

 erties inherent in such a medium. 



A volume of air, under pressure, escapes in the 

 direction of least resistance: and, as there is a con- 

 siderable pressure of the air where the two currents 

 meet, an escape-current nuist form somewhere; and 

 it forms, in accordance with the above law, in the 

 direction of mininuim resistance, obliquely upward 

 to the east or north-east. As soon as this escape- 

 current is developed, its position is at once located by 

 a slender stem of vapor rapidly ascending obliquely; 

 though, if the observer is .at some distance, it seems 

 to be suspended from the cloud above it, and even at 

 times to descend toward the earth. Some of the 

 downward movements of this 'funnel' are apparent 

 only, the constantly ascending vapors sonu'times con- 

 densing near to the ground, and at other limes high 

 in air. The tunnel-cloud is often absorbed into the 



