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Show 1220 REPORT OF THE CHIEF OF ENGINEERS. DESCRIPTION OF METHOD OF MEASUREMENT. Dr. Kampf was aided in the measurement by Mr. Louis Seckels and two laborers. The work was divided among the party as follows: The line was laid out in advance for one day's work, marked by iron pins 2 feet in length and about 180 feet apart. The base was measured on an old road, laid out about ten years ago, and running in a straight line for a distance of 3f miles. A fine line was tied to one pin and fastened to the next one. Two plates were laid down within the distance of 20 feet 2 inches, approximately, so that they were parallel with the line and tangent to it. One laborer takes the measuring- rod, bringing it near the plates, and the assistant being at the rear end, the laborer on the other, both grasp the rod at the same time and put it on top of the plates rA$ of an inch distant from the cross on the ridge of the plates. Mr. deckels reads the rear end of the scales at the same time that Dr. Kampf read8 at the front end by means of a common magnifying- glass. The readings are then at once recorded. After that the level was read by Dr. Kampf and simultaneously with Mr. Seckels, who is now at the front end. He reads the rear end of the scales, and the readings are recorded by both. Then the readings are called out and in case of disagreement repeated. In the meanwhile the other laborer puts an auxiliary rod of 20 feet 2 inches in the position, so that the rear end may be in line with the mark on the plate driving the third plate in the ground. When the readings are finished the new plate is found in its proper position, the laborer brings the rods in front of both plates, and the operation is repeated. The other laborer takes meanwhile the first plate put down and brings it to the front, as No. 4. The thermometer is read from time to time on the shady and sunny side of the rod, to obtain its temperature. After a reasonable practice the rate of measurement may be assumed as 20 feet for each iuterval of one and one- half minutes. CO- EFFICIENT OF EXPANSION. The steel rods of the United States Coast Survey are of normal length at the temperature of 61°. 6 F. By means of the co efficient of expansion for one degree, as given in Lee's tables, the distance of 20 feet is reduced by applying the temperature of the time of comparison, and thereby the distance between both knife- blades is obtained. To this is added the readings of the scales of the rod, and the length of the rod for the observed temperature is found. The mean of the observations at low and high temperatures are taken, and from the difference of both lengths the factor of expansion is derived, as shown in the next table. The rod having been heavily saturated in a rain and snow storm on October 11, the observations taken afterward are not used for determination of expansion. The length of the rod was found from the comparisons to be 20ft 0ln. 9408 for 61°. 6 F. |