IMG76

Ernst A fach

"It should bc mcntioncd that Mach was ablc to explain another ph_c nomcna rclatcd to his ballistic findings which somc scicntists had rc-portcd prcviously in thc literaturę. A meteor falling down to the sur-facc of thc earth produccs first a sharp bang, and only thcrcaftcr one hcars the noisc of thc impact! This bang is nothing morc than thc bow shock reported by Mach, sińce thc meteor comcs down throu^h thc atmosphere with supcrsonic vclocity.”²² Mach’s model law asserted that gcomctrical modcls of projcctilcs having similar shapes but different sizes, if moving at thc same speed, should ideally havc similar shock wavcs and conc anglcs. But thcrc has becn some doubt how rcliablc Mach’s kinematie approach and rcliance on mcrc visual geometry wcrc herc. If the shock wave was smali in size and vclocity it would scem that thc naturę of thc medium, its atomie and molecular structurc, would have to be taken into consid-cration. Of course, Mach did not bclicvc in thc cxistencc of atoms and molecules, and hc tried to avoid having to usc thc atomie theory whcncver possiblc. Nonctheless, thc objcction scems rclevant not only to Mach’s model law, but also to Mach’s wholc approach to ballistics and gas dynamics. Thcrc arc limitations to an ostensibly physical approach which relies almost cntircly on macroscopic obscrvation and visual geometry.

Mach’s collcaguc, Professor Salcher, who carried out cxpcrimcnts for Mach at Fiumc and Pola, attempted an interesting variation on shock-wavc cxperimcnts. Instead of firing a moving piojcctilc through stationary air, why not reversc thc situation and forcc moving air past a stationary projcctile, and this is cxactly what hc did. Dr. Merz-kirch relates: “The expcriments wcrc again performed by Salcher in a torpedo factory in Fiumc; these expcriments arc interesting in so far as this was certainly thc first time that thc idea of a supcrsonic blow-down wind-tunnel had been cxpresscd." "³

By thc late i88os Ludwig Mach was devoting morc time to im-proving his fathcr’s laboratory cquipment than on his own mcdical studies. In short, Ernst Mach was alrcady beginning to lean on the talcntcd laboratory and practical skills of his eldest son, and in this rcspcct he v/as quite fortunate. No father cvcr had a morę dutiful child or useful assistant.

Ludwig Mach worked to improve an instrument, called Jamin’s Intcrfcromctcr, which Ernst Mach had becn employing to obtain lincar graph rcadings of the sizes and presumed strength of shock wavcs.

The twcnty-thrcc-ycar old Ludw.g latcr wrotc: “Concerning the ap-paratus I am dcscribing herc, I had already finished putting its para together by July 1891, but could not complctcly put it in functioning order until Novcmber 1891 bccausc of a dclay in thc arrival of a nccdcd machinę tool.” *⁴ The instrument soon bccamc callcd thc Mach-Zchndcr Intcrfcromctcr and is still used today.²⁵ With this machinę Ernst Mach was 3blc to determine “that thc relative density at thc shock front is about 50 limes what had becn measured for regular sound wavcs." ²⁰ In othrr words, sińce Mach had already cxpcrimcntcd with thc power of acoustic wavcs to break glass, thc suspicion may finally havc occurrcd to him that perhaps shock wavcs wcrc not ncces-sarily so harmlcss after all. If large cnough and with sufhcient vcloc-ity, then we havc that product of thc jct age, a sonie boom.