The author has presented, in the previous paper, theoretical formulae which give the value of virtual mass of water, with regard to vibration of side-walls of a rectangular water tank, full of water. The theoretical formulae were obtained by calculation of kinetic energy of water, which is in vibration induced by the vibration of side-walls. This was an approximate calculation, and not a rigorous one. So, there arose the question about the degree of approximation of the given formulae. In the present paper, this question is examined by taking into account higher modes of vibration of side-walls, and using the minimum principle of Lord Rayleigh. From the result of calculation, we see that the formulae for virtual mass of water in the previous paper are sufficiently accurate for practical use, at least for water tanks having usual proportions.
The formulae for virtual mass, for the case in which the water is only partially filled, is also given here.
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