This ingenious instrument, the invention of Prof. Siemens of London, enables those on board of ships to read from an index the depths of the ocean beneath them. It consists of a highly sensitive steel spring to which a heavy piece of metal is attached. The changes in weight to which the latter is subject in consequence of the variations of attractive force (the deeper the ocean the smaller the latter, and vice versa) are registered on a scale by the indicator that is in connection with the steel spring. What is an Anemometer? An instrument for measuring the velocity and force of the wind, and by which storms, at a distance, can be predicted. What is a Chronometer? A time-piece of delicate and exact construction, chiefly employed by astronomers and navigators. It differs only from an ordinary watch in its delicate springs, in not being so much influenced by heat and cold, and consequently in its accuracy in giving the time. CHAPTER XX. LIGHT, LIME LIGHT, MAGNESIUM LIGHT, ELECTRIC LIGHT, RAINBOW, PRISM, SPECTRUM, COLORS, PHOTOGRAPHY, CAMERA OBSCURA, STEREOSCOPE, KALEIDOSCOPE. Do you know something about the nature of Light? Light is a mere form of vibration like sound, and like sound it requires some source to set this vibration going, and some medium to carry this vibration as air carries sound. Is not the air this medium? No, it is supposed that there is an elastic fluid called "ether" which pervades all space and matter, and if the molecules of a body are in motion they have the power of setting this ether in motion. The movement thus produced will appear either as heat or light according to its velocity. What sources of light do you know? We are told that the principal source of light on earth is the sun, either directly with its own beams or indirectly by supplying us with combustibles to produce light; for oil, gas, candles, and most of the substances used for producing light and heat when burning are but sending forth in another form the rays of the sun which were stored up in nature's economy. Another source of light is the result of chemical action, such as the lime, magnesium, and electric light. A third source of light is phosphorescence, as we see it in the glow-worm and fireflies. What is the Drummond or Lime Light? It is one of the most brilliant of artificial lights. When a stream of oxygen and one of hydrogen under pressure are brought together and mixed within a few inches of the end of a blowpipe, the mixture on lighting burns with a colorless flame possessing intense heat. If this flame be made to play upon a ball of carbonate of lime, the lime on becoming white hot gives off a powerful incandescence. _Incandescence_, the glowing whiteness of a body caused by intense heat. What is a Blowpipe?