Measuring Water Depths.
Frederick J. Smith of Trinity college, Oxford, explains a curious way of finding the depth of a piece of water at a distance:
“About two years ago,” he said, “I wished to know from time to time the rate at which a river was rising after a fall of rain. The river was a considerable distance from the spot where its height was to be known. By means of the combination of two organ pipes and a telephonic circuit described in the following lines, I have been enabled to make the required measurement within rather close limits. At the river station an organ pipe was fixed vertically in an inverted position, so that the water in the river acted as a stopper to the pipe, and the rise or fall of the water determined the note it gave when blown by a small bellows driven by a very small water wheel.
“A michrophone was attached to the upper end of the organ pipe ; this was in circuit with a wire leading to a town station at some distance ; at the town station there was an exactly similar organ pipe, which could be lowered into a vessel full of water while it was sounding. By means of the telephone the note given by the pipe at the river was clearly heard at the town station ; then the organ pipe at this station was lowered or raised by hand until it gave the same note. The lengths of the organ pipes under the water at two stations were then equal, so that the height of the water in the distant river was known.
“ The determination can be made in less than a minute by anyone who can recognize the agreement of two similar notes. The arrangement when first tested was so placed that the height of the water at two places near together might be easily compared. I found that a lad with an average ear for musical sounds was able to get the two heights to agree within oneeighth of an inch of each other, while a person with an educated ear adjusted the instrument immediately to almost exact agreement. The total height to be measured was 17 inches. A difference of temperature at the two stations would make a small difference in the observed heights. For instance, taking a note caused by 250 vibrations per second, a difference of ten degrees C. between the temperature of the two stations (one not likely to occur) would make a difference of about .02 feet in the height, a quantity of no moment in such a class of measurements. The organ pipes were of square section, and made of metal to resist the action of the water.”