The product of these wells after a time greatly diminishes and sometimes ceases altogether. In such cases it is customary to explode torpedoes at the bottom of the well. This is done by placing there several gallons of nitroglycerine with a fulminating cap on top. This cap is exploded by dropping a piece of iron upon it. The explosion opens the seams and crevices around the bottom of the well so as to renew the flow of oil. [Illustration: OIL WELLS.] It is now about forty years since the first introduction of kerosene as an article of commerce. To-day it is in almost universal use throughout the civilized world. It gives a convenient light at a moderate expense, and has therefore proved a great blessing to mankind. Meantime the whale fishery has largely diminished; indeed, it would seem to be almost destroyed. The reasons for this are not difficult to find. In the first place, the number of whales is much less than formerly, so that this business is far less profitable than it used to be. In the second place, the rapid development of the kerosene industry has so cheapened the product that people cannot afford to light their houses with whale oil, especially as they find the kerosene not only cheaper, but more convenient and satisfactory. Common whale oil previous to 1850 had been furnished at an average cost of perhaps fifty cents a gallon, while the sperm oil, which is of superior quality, cost as much as one dollar a gallon. The people of the whole country east of the Rocky Mountains feed their lamps to-day with kerosene at a cost of from eight cents to twelve cents a gallon. A few persons have made great fortunes from the oil wells. On the other hand, it should not be forgotten that the modern processes of purifying kerosene could not have been put in operation without the aid of large fortunes. A cheap and satisfactory light has been furnished to all the people of the United States only by means of the great capital employed in its production. So you see civilization is progressing, and we are all enjoying more blessings and conveniences than our fathers had. In the earlier times every one had to labor diligently to secure food, clothing, and shelter. As civilization advances these require less time and expense, and we have greater opportunities to attend to the development of our higher natures, the acquisition of knowledge, the pursuit of science, and the elevation of the race. CHAPTER V. ILLUMINATING GAS.