“He has,” replied Tom, “and he tells me that it is owing to sound travelling so much more slowly than light.” “You are quite right; and as we are upon an expedition for the purpose of measuring depths, it may not be amiss to inform you, that this fact furnishes another method of calculating distances.” The party seated themselves upon the oak, and Mr. Seymour proceeded--“The stroke of the axe is seen at the moment the woodman makes it, on account of the immense velocity with which light travels;(3) but the noise of the blow will not reach the ear until some time has elapsed, the period varying, of course, in proportion to the distance, because sound moves only at the rate of 1142 feet in a second, or about 13 miles in a minute; so that you perceive, by observing the time that elapses between the fall of the hatchet and the sound produced by it, we can ascertain the distance of the object.” Mr. Seymour fixed his eye attentively on the woodman, and, after a short pause, declared that he was about half a quarter of a mile distant. “Why, how could you discover that?” cried Louisa; “you had not any watch in your hand.” “But,” said Mr. Seymour, “you might have perceived that I placed my finger on my wrist, and as my pulse beats about 75 strokes in a minute, I was able to form a tolerable estimate of the interval, although I confess that it is a very rough experiment, but sufficiently accurate for the purpose of illustration. In the same manner we can readily ascertain the distance of a thundercloud, or that of a vessel at sea firing a cannon. If we do not hear the thunder till half a minute after we see the lightning, we are to conclude the cloud to be at the distance of six miles and a half. But let us proceed to the well.” After a walk of a few minutes, the party reached the place of destination. On their arrival Mr. Seymour enquired who would count the time. “Be that office mine,” said Mr. Twaddleton, as he extracted a large silver time-piece from the dark abyss of his watch-pocket; “and let Tom,” continued he, “find a pebble.” “Here is one,” cried Louisa. “Very well: now, then, how will you proceed?” asked Mr. Seymour. “I shall drop the stone,” replied Tom, “into the well, and observe how many seconds it will be before it touches the water, and I shall then set down the number of feet it will fall in each second, and add up the numbers.” “That,” said Mr. Seymour, “would certainly accomplish your object; but I can give you a neater, as well as a shorter rule for performing the sum: you shall, however, first work it in your own way;--but you have not yet informed me how you propose to ascertain the moment at which the stone reaches the water.” “By the sound, to be sure, papa; and you will find that a very loud one will be produced.”