Kumagai and Itô, since August 7, 1957, have been carrying out the measurements of creep, by bending, of two large beams of granite of a size of 215×12.3×6.8cm. The results obtained in 7, 10 and 13 years were published in this journal: Vol. 14 (p. 507), Vol. 17 (p. 925) and Vol. 20 (p. 185), respectively. Those obtained hitherto in nearly 20 years are given in this paper, which shows that the creep rate today is almost the same as that 10 years ago. And we have come to the conclusion that granite can do a plastic flow with a vanishingly small yield stress, i.e. a viscous flow. The viscosity of granite under the normal temperature and pressure is 3∼6×1020 poises as found from the general trend of the creep.
Itô and Sasajima, since August, 1974, have been carrying out the similar measurements on three granite and three gabbro beams of smaller sizes, 21×2.5×2.0cm and 16×2.0×1.5cm, respectively. In these experiments, an optical flat was set on the upper polished surface of the beam bent convex upward to produce interference fringes of Na-D monochromatic light. By analysing the fringes, the profile of the upper surface was determined to an accuracy of one-tenth of one wavelength. The routine determination of the profile gave a change in the amount of bending with time. Although only 2.5 years have passed, it has been found that the secondary creep of the granite specimens in the general trend is comparable with that found in the previous experiment. As for the gabbro specimens, such a creep has not yet been observed clearly.
During the above two experiments, it has been observed that the creep does not show a steady and monotone progress, but does a repeated“return-back”with an irregular interval of more or less than one year. This strange phenomenon was first noticed after correcting the results of the former experiment for the annual variation of humidity. The latter experiment, being carried out in a constant humidity of neary 100%, shows a similar phenomenon.