When side-walls of a rectangular tank, which is filled with water, are vibrating, the inside water will also make a vibratory motion. This motion of water lowers considerably the natural frequency of vibration of side-walls of the tank. This effect is conveniently expressed by "virtual mass" of water. In the previous reports, I to VI, of the same title, the author has made a theoretical study of the value of "virtual mass," and examined various factors affecting it. These calculations were made by assuming, tentatively, the mode of vibration of rectangular elastic plate which constitute the side-walls of the tank. And, the velocity potential for the vibratory motion of water caused by this vibration of side-walls was found. The question of degree of accuracy of the result obtained by this method, will naturally be raised, and some examination was made in the report II of the same title.
In the present report, this question of degree of accuracy of the author's procedure in the previous reports, is examined. Here, the equation of vibratory motion of a rectangular elastic plate subjected to lateral load (which is the water pressure caused by vibratory motion of water), is taken as the base of our discussion. And, magnitude of correction terms are deduced from it. It is concluded that, at least for rectangular plates of ordinary proportions, the approximate formula given in previous reports may be said to be fairly accurate.