By electrostatic methods, μ-diam spheres of iron have been accelerated to hypervelocities. Techniques have been developed to give single impacts in vacuum of measured incident velocity, mass, and position.

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Preliminarily collision-coalescences between a large water drop and water droplets under free fall were observed. Movements of the droplets due to the effect of the large water drop were also observed. The main experiment was performed by the use of a large water drop and droplets under free fall at terminal velocity, and it was proved that collision-coalescences occurred between them under the condition similar to natural rainfall. The size of the water drop and the size range of the droplets used were 6.2 mm and 80 microns to 200 microns in diameter respectively. Droplets which collided with a drop as they fell in front of the drop were all captured by the drop and no repulsion was observed between them. It was observed that smaller droplets behind a large drop were not captured by the latter because of the wake effect. Furthermore, collisions or coalescences between droplets were not observed within the large drop's wake.

The Lagrange Equations of Motion are written in generalized coordinates which describe small departures from the spherical equilibrium configuration of a conducting liquid droplet. It is initially assumed that the actual shape differs only very slightly from the equilibrium sphere. The equation representing the surface is, then, written as a series of surface zonal harmonics in which the coefficients are shown to be the normal coordinates of the droplet. The frequency of oscillation of the normal coordinates is shown to depend on the total charge on the droplet in such a manner that for all values of charge below a certain limit, the frequency is real. For all values of charge above a certain limit, the frequency is imaginary; and, thus, the droplet is unstable. This paper presents a detailed derivation of a result communicated by Rayleigh in 1882. The results of this communication have been widely quoted but, until now, this particular derivation has not appeared in the literature.

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