Abstract
I. I HAVE to address yon in this course of lectures on what we know of the infra-red end of the spectrum and its relation to solar physics. I will commence by asking a question, and endeavour to answer it in such a way as will, I hope, be understood. The question I propound is, How do we know that there are any rays below the red rays of the spectrum? In answering the question I would beg you to remember that every body in motion possesses what we call energy, or a capacity for doing work, be the motion a wave motion or a direct motion. Let us take one or two examples of waves: first, that of water, which is familiar to us. I need scarcely point out that a wave of the sea is capable of doing an immense amount of work, not to say mischief; there is no doubt, then, that it is capable of doing work, and this we may take as the true definition of energy, existing in a body, viz. the capacity of doing work. Whence, then, does a wave derive its energy? Perhaps we may have to travel many miles from the place where we find our wave. Travelling to the origin of the waves, we shall no doubt find that a wind has generated them, and in reality it is the energy possessed by the wind which is carried by the waves to the distant shore. The energy possessed by the wind has not been directly expended on our coast, but when transmitted by the waves this same energy is applied in different manner, and by this difference in application it becomes effective. We all know, for instance, that a child may ring a church bell if he give a pull at the right intervals of time, and so, by timing the impact of waves correctly, it is possible for them to do work which in any other way would be impossible. Another example of the energy of waves is the tuning-fork, as in the experiment which Mr. Lockyer showed you. You will recollect that he demonstrated that if one tuning-fork was brought near another of the same pitch the second took up the vibration of the air. The tuning-fork which was struck, or bowed, generated waves in the air carrying some part of the energy of the vibrating prongs to be expended on the second tuning-fork, and as this tuning-fork vibrated in the same period as the first one, each blow of the air-waves was essentially well-timed, and the fork was thus set in motion. You will also recollect that a a fork not of the same pitch—that is, not sounding the same note—was unable to cause vibration in the second fork; and this was simply because the energy was applied at wrong intervals of time. In the case of the tuning-fork, then, the air is the medium through which this energy was conveyed.
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Solar Physics 1 . Nature 25, 162–166 (1881). https://doi.org/10.1038/025162a0
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DOI: https://doi.org/10.1038/025162a0