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An experime nt by I\:a nt o J', r ep o rt in g res ul ts in s ha r p contr a dictio n to E instein 's Seco nd Post ul ate, wa s repeated using t he co here nt li g ht of a laser. The res ults were fo un d to be consistent w ith t lw S pecial Th co ry of R elat ivi t.v . It IS co ncluded that t ile ballist ic hy pothes Is of li O'h t pro pagat io n d isrega rdin g t he ef1'rct of ail' is in co rrect. It is poin t ed o ut t hat t he d ire;t experim entn l e vidence in fa vor of E in stein 's Seco nd P ostulate is s ur p ri sin gly meage r a nd f urth er experi ments t o con firm o r rej ect t he ballistic h y pothesis a re envisaged.

Theoretical expressions for the spectral density of amplitude, amplitude-difference, and phase-difference fluctuations of waves propagating over a line-of-sight path in a weakly scattering turbulent medium are derived. Experimental observations made at radio (35 GHz) and acoustic (3 kHz) frequencies are in good agreement with the theoretical predictions under a variety of meteorological conditions. Comparison of experimental and theoretical spectra yields a measure of the average across-the-path wind velocity and the average refractiveindex structure constant C,•. In general, wind speeds and refractive-index structure constants inferred from simultaneous meteorological measurements agree with those obtained from propagation data. rived a theoretical expression for the power spectrum of amplitude fluctuations of a plane wave propagating through a turbulent atmosphere. Later, Tatarskii [1971] extends these results to phase and phasedifference variations; however, he still considers only the plane-wave case. Skrypnik [1966] examines similar spectral expressions for the case of nonuniform wind along the propagation path. Temporal frequency spectra for a spherical wave are obtained by Clifford [1971] and Ishimaru [1972] in the form of double integrals; they also give asymptotic expressions for large and small fluctuation frequencies. Mandics [1971, pp. 17-22] numerically evaluates amplitude and phase spectra for a specific geometry. Numerous workers have calculated fluctuation spectra from measured scintillations of optical, microwave, and acoustic frequencies (e.g., Tatarski [1961], HOhn [1966], Janes et al. [1970], and Golitsyn et al. [1960]). Due to lack of an adequate theoretical framework, however, it has become possible only recently to compare experimentally obtained spectra with the appropriate theory (see Clifford et al. [1971], Ishimaru [1972], and Mandics [1971, pp. 59-74]).The present study will give the results of line-ofsight propagation experiments performed at mm-wave and acoustic frequencies. These experiments are unique in that the incident wavefront is sampled (nearly) simultaneously at several points in space and at a very rapid rate. The sampling rate is set high enough in each case to include all atmosphere-185 of this assumption will have to be examined more closely.Assuming that scatterers of importance lie within the inertial subrange of ,the turbulence spectrum it is possible to express •(•) in the following form [Tatarski, 1961, p. 58] ß ,,(g) = 0.033C,,=(s)t• -•/a 2r/Lo < t• < 2r/go (3) where it is realized that the refractive-index structure constant, C,, may be a function of position SHORT-TERM SIGNAL-FLUCTUATION SPECTRA 157 For the moment, however, the -11/3 exponent indicated in equation 3 is assumed to be valid. It is convenient to normalize the amplitude covariance expression in the following manner' along the path. (Here, l0 and L0 are the inner and that is, it is possible to express the time-lagged autoouter scales of turbulence, respectively.) In addi-covariance function, B•(r), in...

Wind shear has been recognized as a major aviation hazard in the airport environment. A dual, acoustic Dopplermicrowave Doppler radar system has been installed at Dulles International Airport near Washington, D.C., to measure the vertical profile of wind from the surface to 510 m in 30 m height increments. The acoustic system gathers data under clear-air conditions, and the microwave radar takes over automatically when precipitation is present. System performance is being assessed by comparing its output with National Weather Service radiosondes and with balloonborne anemometers and by intercomparing the acoustic-and microwave-measured winds under light precipitation conditions. The dual-sensor system has been operating for several months, registering the passage of fronts, some with potentially hazardous wind shears.

It is pointed out that Einstein's postulate of the constant velocity of light is verified only indirectly by elementary·particle experiments leaning more or less heavily on present e lec tromagn etic theory, the latter being verified only for low velucities. Direct experiments can be explained by the ballistic theory of light if transparent media, such as gases, reradiate as a secondary suurce. A direct experi. nwnt with cuherent light rpAectcd from a moving mirror was performed ill vacuum better than 10-6 torr. Its result is consistent with the constant velocity of light.