By a series of comparisons with Pendulums placed at the surface and the interior of the earth, the Astronomer-Royal has ascertained the variation of gravity in descending to the bottom of a deep mine, as the Harton coal-pit, near South Shields. By calculations from these experiments, he has found the mean density of the earth to be 6·566, the specific gravity of water being represented by unity. In other words, it has been ascertained by these experiments that if the earth’s mass possessed every where its average density, it would weigh, bulk for bulk, 6·566 times as much as water. The immediate result of the computations of the Astronomer-Royal is: supposing a clock adjusted to go true time at the top of the mine, it would gain 2¼ seconds per day at the bottom. Or it may be stated thus: that gravity is greater at the bottom of a mine than at the top by 1/19190th part.[20] The Electric Clock is an invention of our own time. An ordinary clock consists essentially of a series of wheels acting on each other, and carrying round, as they revolve, the hands which mark the seconds, minutes, and hours. The wheels are moved by the falling of a weight, or the unwinding of a spring; and the rate at which they revolve is determined by the length of a pendulum made to oscillate by the wheels. In electric or (as they should rather be called) electro-magnetic clocks, there are neither weights nor springs; so that they never run down, and never require to be wound up. To produce motion, electricity is employed alternately to make and remake an electro-magnet, or alternately to reverse the poles of a permanent magnet, which, by lifting up and letting fall, or attracting and repelling a lever, moves the wheels. M. Bouilly endeavoured to show that character was much influenced by Time-keepers. He describes two young persons who were allowed to select Watches for themselves: one chose a plain watch, being told that its performance could be depended on; the other, attracted by the elegance of a case, decided upon one of inferior construction. The possessor of the good Watch became remarkable for punctuality; while the other, although always in a hurry, was never in time, and discovered that, next to being too late, there is nothing worse than being too early. The choice of a good Watch is, however, a difficult matter: none but a good workman is capable of forming a correct opinion; and a Watch must be bad indeed for an inexperienced eye to detect the errors either of the principle or its construction; even a trial of a year or two is no proof, for wear seldom takes place within that time; and while a good Watch can but go well, a bad one, by chance, may occasionally do so.