IMG73

Ernst Mach

of notoriety from atomie bomb cxplosions whcrc it is often uscd to refer to the rcsulting ground-lcvcl shock \vavcs. On the whole, the notions of Mach ctfcct and Mach rcflcction havc becomc important i_n gas dynami es, cvcn though they set in at angles somewhat less than thosc prcdictcd by theory.¹¹

In 1878 Mach attempted to follow up the success of his soot expcri-ments by using the so-callcd Tocplcr striation method to get a elear photograph of what shock waves lookcd likc. But their cxtrcmc speed, beyond the speed of sound, madę this a vcry formidablc undertaking. It was one thing to infer the wavcs from soot formations and quite another to get a sharp photograph picturc of them.

Mach was cxtremcly busy during the ncxt six ycars with two rcc-torates, the writing and publication of two books, scattcrcd explosion cxpcriments, and work in clcctricity. The lattcr field had suddenly be-come very popular at least among physicists, but Mach’s contributions in it were not very important and stood in depressing contrast to thosc of his old Vienna rival Joscf Stefan, now president of both the Austrian Elcctro-Technical Union and the Vicnna International Electric Fair (1883). In addition, Stefan had also becomc known as the dircctor of the best-run physical institutc on the Continent.

diacram 5. “Y” Diagram. Irregular or Mach rcflcction of a dircct shockwavc on an anglcd wali:

(A) primary shock wavc, (B) Mach stem,

(C) part of the regular rcflccted wavc, and (D) triplepoint.

III

Mach’s first scrious aitcmpt to icst Mclscns’s hypothescs on the naturę of shock wavcs in front of projcctiles took place in 1884. He started by trying to takc cxact photographs of four different kinds of speeding phenomena: flying projcctiles, sound wavcs, spark wavcs, and projcctilc shock \vavcs.

Rcasonably successful attempts to photograph rr.oving projcctiles had bccn madc as carly as 1866 in Woolwich, England, and again in 1S82 by Marcy in France, but Msch’s 1884 picturcs represented a sub-stantial improvemcm in darity and distinetness. Mach was also suc-ccssful in taking good picturcs of sound and spark wavcs in 1884. With rcspcct to projcctile shock wavcs a Frcnchman, Captain Journćc, elaimed that he had scen one by using a tclcscopc down a projcctilc’s linc of flight, and on that basis he elaimed prcccdcncc.¹"

In this same ycar, 1884, Mach began his major effort to photograph projcctilc shock wavcs. By this timc Mach had learned how to circum-

diagram 6.

Tocplcr’s Schliercnmcthode. An apparatus rcconstructcd by Ludwig Mach to photograph the shock wavc in front of a projcctilc without also having to photograph a trip wire.

{Co tir U ty of HolJcn-Day Incorporatcd)

109