Before passing on to consider upward movements of a more rapid nature, such as are frequently caused by earthquakes, we may pause for a few moments to consider certain very slight, but nevertheless very interesting little movements, such as _slight pulsations_ and tremors, which have been observed to take place in the earth's crust (as it is called), and which of late years have been carefully studied. Professor Milne, a great authority on earthquakes, has noticed slight swayings of the earth, which though occupying a short time--from a few seconds to a few hours--are still too slow to produce a shock of any kind. These he calls "earth pulsations." They have been observed by means of delicate spirit-levels, the bubbles of which move with very slight changes of level at either end of the instrument. At present only a few experiments of this kind have been made; but they tell us that the surface of the earth (which is apparently so firm and immovable) is subject to slight but frequent oscillations. Some think that they depend upon changes in the weight of the atmosphere. If this is so, the balance between the forces at work below the earth's surface and those that operate on its surface must be very easily disturbed. Still we cannot see that this is a serious objection; on the contrary, there is much reason to think that any slight extra weight on the surface, such as might be caused by an increase of the pressure of the atmosphere, and still more by the accumulation of vast sedimentary deposits on the floor of the ocean, may be quite sufficient to cause a movement to take place. Moreover, Mr. G. H. Darwin has shown that the earth's crust daily heaves up and down under the attraction of the moon in the same kind of way that the ocean does; so that we must give up all idea of the solid earth being fixed and immovable, and must look upon it as a flexible body, like a ball of india-rubber (see chap. ix., pp. 314-315). Slight movements of rather a different kind have been noticed, to which the name of "earth-tremors" has been given. These are very slight jarrings or quiverings of the earth, too slight to be observed by our unaided senses, but rendered visible by means of very delicate pendulums and other contrivances. Now wherever such observations have been made it has been discovered that the earth is constantly quivering as if it were a lump of jelly. In Italy, where this subject has been very carefully studied, the tremors that are continually going on are found to vary considerably in strength; for instance, when the weather is very disturbed and unsettled, the movements of the pendulum are often much greater. Again, before an earthquake the instrument shows that the tremors are more frequent and violent.