344 The Realm of Nature 



decimally. The unit is the metre (about 39^ inches), divided into 

 10 decimetres or 100 centimetres or 1000 millimetres ; and for 

 measuring long distances 1000 metres are called a kilometre. It is 

 convenient to remember that 25 millimetres are nearly equal to I 

 inch, or, more exactly, that 33 centimetres are equal to 13 inches, . 

 and that 8 kilometres are equal to 5 miles. The measures of volume 

 fluid ounces, pints, gallons, bushels, cubic inches, cubic feet, used 

 in English-speaking countries are as confused as the other standards, 

 while the unit volume of I litre (about if pints) divided into 1000 

 cubic centimetres is as convenient as the other parts of the metric 

 system. The only connection between the British systems of 

 weights and measures is that the gallon is fixed as the volume of 10 

 Ibs. of pure water at 60 F. Relations of a much more intimate 

 kind pervade the metric system. It is true that the metre is not 

 quite the length originally intended, which was nro'troooo f a 

 quadrant of the Earth's meridian, but the litre is a cube I decimetre 

 in the side, and the kilogramme is the mass of I litre of pure water 

 at 4 C. , the gramme being similarly equal to the mass of I cubic 

 centimetre of water at the maximum density point. Notwithstand- 

 ing the simplicity and convenience of the metric system, it was 

 considered advisable in this book to make use of the familiar British 

 units in order to present the facts of science in the manner most 

 easily grasped by English-speaking people. 



438. The Mariner's Compass consists of a magnetised steel needle, 

 or a series of such needles fixed parallel to each other, delicately 

 pivoted in a box, which is loaded with lead and hung so as to remain 

 horizontal in spite of the tossing of a ship. A light circular card is 

 fixed above the needles and moves with them. The point over the 

 north-seeking end of the needle is marked as the North, the opposite 

 point is marked South, and the ends of the diameter at right angles 

 East and West. The edge of the card is divided into 360 degrees, 

 there being 90 in each quadrant, i.e. from N. to E. or from E. to S. 

 The exact direction or bearing of a distant object may be stated as 

 N. 45 E. if it appears midway between the north and east points of 

 the horizon as estimated from the card. Sailors have another way 

 of expressing direction. They divide the edge of the card into 

 thirty-two "points," each containing u| degrees, but divided into 

 halves and quarters. For each point they have a special name ; thus 

 the quadrant from north to east is divided into North, North by 

 East, North- North- East, North-East by North, North-East, North- 

 East by East, East- North- East, East by North, East ; and so on 



