50 The Realm of Nature CHAP. 



the Earth is hence generally spoken of as being a sphere or 

 globe. From 5 feet above sea-level the horizon is only 

 3 miles distant ; from a height of 4000 feet it is 80 miles, 

 so that an observer can see to a distance of 80 miles 

 all round j while from 24,000 feet it is more than 200 

 miles distant, and in each case a perfect circle. 



82. The Earth an Ellipsoid. If the Earth were a 

 perfect sphere its size could be measured by measuring the 

 length, in miles or yards, of the arc of a great circle (i.e. a 

 circle the centre of which is at the centre of the Earth) sub- 

 tending one degree, and multiplying by 360 to give the cir- 

 cumference, for each degree subtends an equal arc on a 

 sphere. It is easy by observations of the stars ( 92) to tell 

 exactly how many degrees one has advanced along a great 

 circle ; and parts of great circles (arcs of the meridian) have 

 been measured in many parts of the Earth with much exact- 

 ness. In Great Britain i was found to be almost exactly 

 365,000 feet long; but in Peru i was found not quite 

 363,000 feet in length, and in the north of Sweden i was 

 found to measure about 366,000 feet. These measurements 

 are undoubtedly correct to within a few feet ; and the only 

 conclusion that can be drawn from them is that the Earth 

 is not a sphere, but a figure the curvature of which is less 

 than that of a sphere in some parts and greater in other 

 parts. It resembles a sphere slightly compressed along one 

 diameter, and correspondingly bulged out in the direction at 

 right angles. The length of the shortest diameter has been 

 calculated as 7899-6 miles (about 500,000,000 inches), and 

 the diameter at right angles as 7926-6 miles. The circum- 

 ference is about 24,000 miles. The form is very nearly 

 that known as an ellipsoid, or oblate spheroid of revolution 

 a figure that could be made in a turning-lathe, with the 

 axis of rotation in the lathe as the shortest diameter. 



83. The Earth a Ball. The most exact measurements 

 which have been made, show that the figure of the Earth is 

 not a true ellipsoid. It appears to be compressed to a slight 

 extent at right angles to the shortest diameter, so that the 

 equatorial diameters vary in length by one or two miles. 

 The exact form of the Earth is being gradually discovered 



