30 UNITED STATES COAST AND GEODETIC SURVEY. 



20-foot scale, 1 to 30, 1 to 40, or 1 to 60, and with amplitudes at their 

 true value and the use of the 40-foot scale, 1 to 60, 1 to 80, or 1 to 120 

 of nature. 



Tide curve. — The tide curve, though traced incidentally to the 

 regular predictions for use as a record, also serves another purpose. 

 The tides at some stations are of the form in which at times the 

 diurnal overcomes the semidiurnal wave, thus producing vanishing 

 or evanescent tides (plate 15, at a), in which frequently a theoretical 

 high or low water is lower or higher than the preceding low or high 

 water, respectively. In such cases it becomes necessary, in order 

 to avoid cumbering tfce tide tables with theoretical data, to make 

 such compromises as are most suitable for the practical needs of the 

 mariner. This can be done by an experienced operator by inspec- 

 tion of the curve, of which more than a day is exposed at all times, 

 as the predictions proceed, so that, when the high and low waters of 

 the last day of the year have been ^vritten down, the printer's copy 

 for the station is completed. 



On plate 15 are shown two facsimile curves on actual scale produced 

 by the machine. One of them shows, at h the manner in which the 

 small difference in time between the curve and base line pens is 

 ascertained. 



Automatic stopping device. — For relieving the operator of the greater 

 part of the strain due to watching the appearance of the platinum 

 zero link in the time chain, and stopping the machine at the instant 

 of its coincidence with the index, an automatic stopping device is 

 provided. It consists of an electric circuit which, when closed, 

 causes an electromagnet mounted under the desk top (plate 8) to 

 throw down upon the edge of a ratchet wheel, 5J inches diameter, 

 400 teeth, | inch face, secured to the crank shaft, a steel pawl, thereby 

 arresting the motion of the crank and stopping the machine. The cir- 

 cuit is closed by a contact spring which rests upon a hard-rubber cyl- 

 inder (plate 9) on the rear end of the shaft holding the puUey upon 

 which the time chain ends. A small platinum plug in this cylinder 

 comes in contact with the spring, which latter is fitted with a fine 

 motion adjustment, when the zero link of the time chain is in coinci- 

 dence with the index. The lateral screw motion of this shaft prevents 

 the platinum plug from again making contact with the spring when 

 the shaft has made one or more revolutions on either side of the zero 

 position of the chain. The circuit is led through an insulated ring 

 on the hub of the crank and a contact kept closed by a spring. A 

 slight inward pressure against the crank handle overcomes this 

 spring, breaks the circuit, and releases the armature and pawl, thereby 

 permitting the machine to be moved forward to the next stop. By 

 means of a smaU switch just below the crank the battery can be cut 

 out. 



