shall take the form of heat or give forth an electric current. In the former case he shall connect the receivers together by suitable tubes, apply a spark, and obtain a flame hot enough to fuse and vaporize iron or platinum. In the other case he can use Grove's gas-battery, and permit the elements to unite into water, producing an intense electric current capable of working scores of miles of telegraph. The realm of light yields us examples analogous to those given in the domains of heat and electricity. In photography it has been discovered that blue rays may begin an impression which red or yellow ones can finish, and finish only. The power of continuance is different from the power of initiation, and depends upon it for its opportunity of usefulness.
The instability of equilibrium among forces brings in an element of uncertainty, or rather incalculability, which renders prediction extremely difficult in many fields of scientific investigation. Prof. Balfour Stewart, in a most instructive essay on "Solar Physics," gives us some illustrations of this. He supposes a stratum of air in the earth's atmosphere to be very nearly saturated with aqueous vapor; that is to say, just a little above the dew-point; while at the same time it is losing heat with extreme slowness, so that if left to itself it would be a long time before moisture were deposited. Now, such a stratum is in an extremely delicate state of molecular equilibrium, and the dropping into it of a small crystal of snow would at once cause a remarkable change of state. For what would happen? The snow would cool the air around it, and thus moisture would be deposited in the form of fine mist or dew. Now, this deposited mist or dew, being a liquid, and as such much more radiant than vapor, would send its heat into empty space much more rapidly than the saturated air; and therefore it would become colder than the air around it. Thus, more air would be cooled, and more mist or dew deposited; and so on until a complete change of condition should be brought about, resulting perhaps in a shower of rain. Now, in this imaginary case, the tiniest possible flake of snow has pulled the trigger, as it were, and made the gun go off—has changed completely the whole arrangement that might have gone on for some time longer as it was, had it not been for the advent of the snow-flake. Prof. Stewart thus points out that the presence of a condensable liquid in our atmosphere adds an element of violence, and also of abruptness, amounting to incalculability, to the motions which take place. Hence meteorology must long, if not ever, remain an incomplete science, since in its problems so many variable and unstable factors occur. In the course of the same essay Prof. Stewart tells us how parallelism has been observed between three very interesting classes of phenomena, namely, the periods of maximum sunspots, of brilliant auroral displays, and of great disturbances in the earth's magnetism. Extended observatory records show that all three coincide in their fluctuations; hence endeavors have been made to
