. Scientific American Volume 86 Number 05 (February 1902) . heories could be verified experi-mentally much would be done both for hydrodynam-ics and aerial dynamics. Furthermore, it would bepossible to devise a body which would encounter theleast possible resistance in moving through water orair; for the laws underlying the movement of bodiesaround a stationary obstacle would apply converselyto a body moving through a fluid. The first attempts to observe the movements offluids directly by the eye, or indirectly by means ofphotography, were made some years ago by L. these classicexperim
. Scientific American Volume 86 Number 05 (February 1902) . heories could be verified experi-mentally much would be done both for hydrodynam-ics and aerial dynamics. Furthermore, it would bepossible to devise a body which would encounter theleast possible resistance in moving through water orair; for the laws underlying the movement of bodiesaround a stationary obstacle would apply converselyto a body moving through a fluid. The first attempts to observe the movements offluids directly by the eye, or indirectly by means ofphotography, were made some years ago by L. these classicexperiments ofhis, the warmand cold airwas admittedinto an obser-vation cham-ber by way ofnumeroussmall threads ofair streamingthrough theseopenings con-tinued theirmovement i nthe observationchamber with-out intermin-g 1 i n g. Al-though the eyecould see noth-ing of this phe-nomenon, thephotographicplate showedthat different-ly heated cur-rents did notmingle; for itis a well-knownfact that lightdoes not travelwith the samespeed in cold and warm air. M. MAREYS APPARATUS FOR STUDYING AIR CUBRENTS. By means of this apparatus Hele-Shaw was enabledto produce photographs corresponding exactly withthe theoretical forms of current lines for fluids mov-ing in narrow passages, pictured in Lambs Hydro-dynamics. This mechanical method, however, is de-fective in so far as it shows only the direction of Still another methodof studying the current lines of fluids was adopted byan English physicist, Hele-Shaw. In 1897 he beganto study the motion of water circling between the twoparallel glass walls of a vessel and encountering var-ious obstacles. Accidentally he found that a mixtureof air and water, by reason of the division of the airinto a number of globules, rendered it pos-sible to follow the motion of the water withconsiderable accuracy. Photographs showednot only the places where vortices were pro-duced by reason of the obstacles, but alsoproved the general law that in all cases,
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