Washington, desired that I should lecture in some of the principal
cities of the Union. This I agreed to do, though much in the dark as
to a suitable subject. In answer to my inquiries, however, I was given
to understand that a course of lectures, showing the uses of
experiment in the cultivation of Natural Knowledge, would materially
promote scientific education in this country. And though such lectures
involved the selection of weighty and delicate instruments, and their
transfer from place to place, I determined to meet the wishes of my
friends, as far as the time and means at my disposal would allow.


§ 2. _Subject of the Course. Source of Light employed._

Experiments have two great uses--a use in discovery, and a use in
tuition. They were long ago defined as the investigator's language
addressed to Nature, to which she sends intelligible replies. These
replies, however, usually reach the questioner in whispers too feeble
for the public ear. But after the investigator comes the teacher,
whose function it is so to exalt and modify the experiments of his
predecessor, as to render them fit for public presentation. This
secondary function I shall endeavour, in the present instance, to
fulfil.

Taking a single department of natural philosophy as my subject, I
propose, by means of it, to illustrate the growth of scientific
knowledge under the guidance of experiment. I wish, in the first
place, to make you acquainted with certain elementary phenomena; then
to point out to you how the theoretical principles by which phenomena
are explained take root in the human mind, and finally to apply these
principles to the whole body of knowledge covered by the lectures. The