“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.”
“If the depth of the well be considerable, such a plan will not answer the purpose, since, in that case, there must necessarily be a perceptible interval between the fall of the stone and the sound produced by it, as you have just seen exemplified by the woodman, which, unless taken into account,[(4)] will vitiate the result.”