4544 



HYDROGRAPHIC MANUAL 



Page 382 



4544. Stop Watch 



A stop watch "has many uses, both m navigating and surveying, and its use is 

 essential for measuring small intervals of time accurately. It is used in current obser- 

 vations, in checking the speed of echo-sounding instruments, and in R.A.R. for timing 

 the ship's run between the instant a bomb strikes the water and its explosion. In 

 navigating and piloting it is useful in measuring wind velocity with an anemometer, in 

 counting the revolutions per minute of the ship's propeller, in taking celestial sights, in 

 measuring the distance from shore by an echo, and in measuring the distance from a 

 synchronized radiobeacon and sound-signal station. And it can be used to advantage 

 in many other operations. 



455. Rangefinder 



A rangefinder is an instrument with which the distance of an object may be 

 measured with reference to a short base incorporated in it. In its simplest form it 

 consists of a tube within which are two prisms at the ends of the base, arranged to bring 

 reflected images to a central eyepiece into which the observer looks. The distance 

 between the prisms is the base and the instruments are usually classified by the base 

 length, e. g., a 1-meter rangefinder. The over-all length of the instrument is slightly 

 greater than the distance between the prisms. 



A distance is measured by looking into the eyepiece and bringing the two reflected 

 images of the object into coincidence by an adjustment provided to move one with 

 reference to the other. The distance may then be read on a graduated scale which is 

 operated by the mechanism for bringing the images together. The scale is graduated 

 in either meters or yards; which should be ascertained before using a particular 

 instrument. 



The rangefinders used in the Coast and Geodetic Survey generally have base 

 lengths of either 30 centimeters or 1 meter, the smaller being used by launch and 

 shore parties and the larger on board the survey ships. 



The range and accuracy of a rangefinder vary with the length of base and preci- 

 sion of manufacture. The 1-meter rangefinder is graduated from 250 to 20,000 yards, 

 and the 30-centim.eter rangefinder from 20 to 1,000 meters. Table 10 indicates the 

 accuracy to be expected with each instrument. 



Table 10.- — Probable error of rangefinder distances 



