378 Introduction to the Study of Science 



Dead reckoning. In prolonged cloudy weather navigators 

 depend upon what they call " dead reckoning." They know 

 the general directions by means of the compass. They have 

 the rate of speed as shown by the log. With the data thus 

 furnished they can compute the distance traveled, and conjec- 

 ture the position of the ship as to latitude and longitude. They 

 allow roughly for variable winds, the influence of ocean currents, 

 and possible error of the compass ; and are able to locate the 

 vessel closely enough for practical purposes. 



SUMMARY 



The compass depends upon the action of a magnetic needle in the 

 magnetic field of the earth. 



The angle that a magnetic needle makes with the meridian is the 

 decimation or variation of the needle. It varies slightly from year to 

 year. 



The magnetic compass is unreliable when used on steel ships, 

 which are themselves magnetized by the earth. This has led to the 

 use of the gyro-compass. 



The chronometer is an accurate clock, the daily error of which is 

 known. It keeps London time and is used in determining the longi- 

 tude of the ship's position. 



Latitude is determined by observation of the sun, or other heavenly 

 body, made by means of a sextant. 



The log is an instrument dragged through the water for the pur- 

 pose of determining the distance the ship travels and its speed. 



Determination of the ship's position from a former position, by 

 means of the log and compass, is called dead reckoning. 



REVIEW QUESTIONS 



1. (a) What are the instruments of navigation necessary to a sail- 

 ing vessel? (6) Which of these were used by Columbus? 2. What 

 was the relation of the compass to the geographical discoveries of the 

 fifteenth century? 3. What peoples made early use of the compass 

 and for what purpose? 4. (a) What is a lodestone? (6) Mention 

 some of its peculiar properties. 5. (a) How may a simple compass be 

 made? (6) Describe the mariner's compass, (c) What sort of box 

 is unsuitable for it ? 



6. Does the compass in most parts of the earth point to the true 



