[Footnote 23: It is supposed by many, that when the spark is long enough we can note the direction in which it travels; and observations of the motion of lightning from the earth to the cloud have been collected, as showing that the usually observed direction of the flash is sometimes reversed. In reality, no one has ever seen a lightning flash travel either one way or the other. If the attention is fixed on the storm cloud, as usual when a lightning storm is watched, every flash appears to pass from the cloud to the earth. If, on the contrary, at the moment when the attention is fixed on some terrestrial object the lightning flashes near that particular object, the flash will seem to pass from the object to the cloud. In either case the motion is apparent only. If there is motion at all, the passage of the electric spark occupies less than the 100,000th part of a second, and of course it is utterly impossible that any eye could tell at which end of its track the flash first appeared. In every case the flash seems to travel from the end to which attention was more nearly directed. The apparent motion corresponds to the chance direction of the eye.] [Footnote 24: The extremity of the wire connected with the metal least affected by the acid solution is called the positive pole, that of the wire connected with the metal most affected by the solution is called the negative pole.] [Footnote 25: So called, though in reality the best magnets gradually lose force.] [Footnote 26: My occasional use of the word 'heat' where in scientific writing 'temperature' would be the word used, has exposed me to peevish, not to say petulant comments from Professor P.G. Tait, who has denounced half the mathematical world for using the word 'force,' in the sense in which Newton used it, and has spoken of an eminent physicist as one deserving universal execration and opprobrium for not explaining, when speaking of work done against gravity, that terrestrial gravity was meant, and not gravity on the sun, or Jupiter, or Mars, or anywhere in the heavens above or in the earth beneath, but only at the earth's surface. Where there is no risk of confusion, the word 'heat' may be used either to signify temperature, as when in ordinary speech and writing we talk of blood-heat, fever-heat, summer-heat, and so forth. Science, indeed, very properly forbids the use of the word in any sense save one. But outside the pages of scientific treatises, there is no inaccuracy in using a word in a sense popularly attributed to it, when no mistake can possibly arise. No one can suppose, when I speak of a heat of so many degrees Fahrenheit or Centigrade, that I mean anything but such and such a degree of heat, any more than if I spoke of the intense heat of that _savant entêté_, Professor P.G. Tait, any one would imagine that I referred to his calorific condition.]