IMG74

Ernst Mach

vent a major obstaclc. Bcsidcs being £ast and smali, shock wavcs _Wcre normally invisiblc, that is, sincc thcy consistcd of air thcy wcrc trans" parcnt. Mach’s solution was to usc an illuminatcd magnłfying l_cns which would make thc water condcnsation within thc shock wave vis iblc as a shadow contrast.¹³

Profcssor Floyd Ratliff has dcscribcd how thc arrangement (_scc diagram 6) workcd:

Mach dcsigncd an improvcd version ot thc [Tocplcr] apparatus in which thc projcctilc, passing through a ring, forccd air through a smali tubc-thc cscaping air causcd a candlc flamc ai thc cnd of thc tubc to wavcr The flamc in turn shortcd a circuit, causing a Icydcn jar to dischargc and producc a hricf spnrk for illumination of thc projcctilc. By an adjustment in thc length of thc tubc, thc dischargc could l>c set off at thc moment thc projcctilc rcachcd thc center of thc field covcrcd by thc camera.¹⁴

Mach first tried a target pistol and soon obtaincd cxccllcnt photo-graphs of the hurrying bullcts, but without any accompanying shock wavcs! Wherc wcrc thcy? Had hc donc something wrong? Mach soon rightly suspectcd that the vclocity of thc pistol bullcts had not cx-cecded the speed of sound. Thcir speed was only about 240 meters per second whcrcas thc speed of sound at that altitude was about 340 meters per second. Another cxpcriment had failed. The shock waves remained elusivc. About two years later after finishing his book on psychology and having completed a number of additional cxperiments on spark and cxplosion wavcs, hc returned to the problem. This timc hc was determined that thc projectiles would move faster than thc normal speed of sound. Mach wrotc to Professor P. Salcher of thc Austrian Naval Academy in Fiurnc on thc Adriatic in early 1886. He was ablc to persuade him and a Professor Ricgler to conduct balii Stic cxperimcnts under closcly written instructions from Mach. Over a hundred letters from each sidc wcrc exchanged during thc ncxt year, as advicc, analysis, corrections, and reports were communicated back and forth.

Success finally came, and on Junc 10, 1886, Mach deposited a short noticc with thc Austrian Acadcmy of Science along with thc first two photographs of projectile shock wavcs cver succcssfully taken.¹⁸

The smali but rcspcctcd Austrian Navy (undefeated in its only flect action using armored ships, i.c., against Italy in 1866) was pleascd with Mach’s and Salcher’s rcsults and soon madę a largc cannon avail-ablc to thc lattcr at Pola on thc Adriatic. Not to be outdonc, Prussia,

an ally o f Austria-Hungary, through Hcrr Krupp of thc famous stccl-and munitions-producing faniily, otfcrcd Mach himsclf a location at Mcppcn in Hamiovcr to continuc his cxpcriments. Mach and his son Ludwig, thcn a student of mcdicinc, acccptcd thc offcr. Mach’s photo-graphs wcrc wcll publicizcd at thc timc, cvcn in popular magazines and ncwspapcrs.

In spite of public recognition and thc generosity of the Naval Department and Krupp, Mach soon camc to a rather practical conclusion: “AU these cxperiments [at Pola and Mcppcn] furnished tolcrably good and complete pictures; sonie littlc progress, 100, was madę. The out-comc of our cxpcriencc on both artillcry ranges, howcvcr, was thc scttled conviction that really good results could be obtained only by the most careful conduct of thc experiments in a laboratory cspecialiy adapted to thc purpose.” ¹⁰

With his first projcctilc shock wavc photos Mach could now conftrm the suspicions of artillcry ofłicers as to why two reports wcrc often hcard after a gun was fired. The first report was from thc accompany-ing head shock wavc which was faster than the speed of sound, and the second report was from slower but normal sound waves coming from the exploding powder. If only one report was hcard thcn pre-sumably the projcctile must have bcen traveling slower than the speed of sound.

Mach also discovcred that thc head wavcs rcsembled the front or bow wavc$ of ships and could largcly bc explained as Christian Huy-gens had done with acoustic wavcs some two hundred ycars beforc.

Mach now fclt compctcnt cnough to refute two of Mel$ens’s hypoth-eses. First, projectiles did not carry air masses along with them, either in front as with Mclscns or behind as with Aristotle, but rather, con-secutivc dynamie air disturbances of rclatively stationary air gavc the impression of a continuing motion of the same air. Second, the projcctile head waves were so thin as to have ncgligiblc vclocity against objeets, ccrt.iinly not cnough to be responsiblc for significant aspects of battlc wounds. In Mach’s own words:

A forward-moving sound [i.c., shock] wave is not a forward-moving mass of matter, but a forward-moving form oi motion, just as a watcr-\vavc or thc wavcs of a field of wheat arc only forward-maYing fornis ot motion and not movemcnis of masses of water or masses of wheat, . . . it was found that thc bcll-shaped hcadwase in question is an cxtrcniclv thin shell and that thc condcnsaiions ol the same arc quite moderate. scarccly c\eccd-ing two tcnths of an atmosphcrc. Thcrc can bc no ąuestion, thacforc. oi