The marine record shows that over the last 350 ka Northern Hemisphere ice sheet volumes have fluctuated widely and only on rare short occasions have they been reduced to the present interglacial state. The fluctuations are well synchronized with hemispheric average summer insolation variations of 20 ka periodicity caused by changing orbital parameters. The development of a model which explains the varied amplitudes of the fluctuations and is consistent with the geological record embodies the following arguments: The transition from an interglacial state like today's to a glacial state is initiated when a summer insolation deficit causes a southerly extension of the North Atlantic-Arctic pack ice to 60°N latitude. The extension alters the subpolar low pressure patterns and thus causes a southward diversion of the European Gulf Stream flow. It also produces an enhanced warm West Greenland current. This current causes open seas as far north as Baffin Bay which provides moisture for rapid northern Laurentide ice sheet growth. After several glacial fluctuations driven by insolation variations, the southern Laurentide ice front may reach an extreme extension. This diverts the westerlies and the Gulf Stream thus weakening a dominant subpolar North Atlantic gyre and consequently producing a prolonged cutoff of the West Greenland current and a reduction of high latitude glacial precipitation. The subsequent high insolation can then melt back the eastern pack ice and restore the northern European Gulf Stream. This warms the high latitudes for a time sufficient to melt the continental ice, thus causing the transition back to the interglacial state.An analysis of the record in the context of model suggests that the threshold deficit in average summer insolation that is required to initiate major glacial growth is influenced by the cooling effect of the Greenland ice cap on the seas to the east. The threshold level under conditions like today's is found to lie between −7 and −17 ly/day relative to the present. This threshold will not be crossed for at least 54 millenia due to an interval of smaller orbital eccentricity. Probable melting of the Greenland ice cap about 30 ka AP would ensure the extension of the present interglacial beyond 120 ka AP.

A pulsed probe has been used to measure certain parameters of a time varying plasma in mercury vapor. Langmuir probe characteristics have been obtained by pulsing the probe voltage to successively higher values for μsec intervals at specific times relative to a repetitive discharge pulse. Curves showing the time dependence of the plasma potential, electron temperature, and electron density relative to this pulse are presented. The probe characteristics do not exhibit a sharp break at the plasma potential. This is related to a disturbance of the plasma by the probe. A novel and precise technique for determining the actual plasma potential is described. This technique depends upon the abrupt appearance of a spike on the leading edge of the probe current pulse.

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The diffusion of particles from the center of a sphere of radius R is considered theoretically. An expression is obtained for the probability that a particle will be collected at a small probe within the sphere rather than at the surface. This result forms the basis for an experiment to determine the momentum transfer cross section Q m for electrons in a gas. The only parameter which must be known in order to obtain absolute cross sections by the technique proposed is the acceptance coefficient for electrons at the probe. The probability A c that a particle will have exactly c collisions characterized by a constant cross section Q before striking the surface of the sphere for the first time is evaluated. It is assumed that these collisions are not correlated with the diffusion process. In contrast with previous work the present method takes into account the variation of total diffusion time as well as the probability of a particular number of collisions during a given time. The probabilities Ao, A\, and A2 are given in terms of elementary functions and a recursion relationship is given for the other A's. The expression A 0 = y csch y (where y -RN{5QQ m ) 112 and N is the gas density) is useful in an experiment to determine the cross section for inelastic collisions between electrons and gases. The probability of more than any given number of collisions, the average number of collisions, and the mean-square deviation from this average are also evaluated.

Observations of the electron density, visible and near ultraviolet light intensity, and spectrum associated with a 3000 Mc/sec. pulsed electrodeless discharge have been made. Our electron density data indicate recombination-type electron removal with a constant a = 1.0XlO~8 cm 3 /ion-sec. in reasonably good agreement with previous data. During the discharge, the spectrum of atomic helium predominates. In the afterglow, the band spectrum of He2 predominates. The total energy radiated in the wave-length range 2000 to 8000A is estimated as 0.5 electron volts per electron removed. A maximum in the light intensity occurs a few hundred microseconds into the afterglow. The following mechanism is quantitatively consistent with all of our data: He + forms He2 + by undergoing a triple collision with two neutral atoms, after which the molecular helium ions combine with electrons. * L. Herman, Comptes Rendus 228, 2016 (1949).