ITS DISTANCE. 



the mortar. Yet the building is inaccessible to him ; the walls 

 of the town, the fortifications, and perhaps a river, intervene. 

 He finds, however, no difficulty in measuring the distance of this 

 inaccessible building. To accomplish this, he lays down a space 

 upon the ground he occupies, called the base line, from the 

 extremities of which he takes the bearings or directions of the 

 building in question. From these bearings, and from the length 

 of the base line, he is enabled to calculate by the most simple 

 principles of geometry and arithmetic the distance of the building 

 in question. Now imagine the building in question to be the sun, 

 and the base line to be the whole diameter of the globe of the earth, 

 in what respect would the problem be altered? The building within 

 the town is inaccessible so is the sun; the base line of the 

 engineer is exactly known so is the diameter of the earth ; the 

 bearings of the building from the ends of the base line are known 

 so are the bearings of the sun's centre from the extremes of the 

 earth's diameter. The problems are, in fact, identical ; they differ 

 in nothing except the accidental and unimportant circumstance of 

 the magnitudes of the lines and angles that enter the question. 

 In short, the measurement of distances of objects in the heavens 

 is effected upon principles in all respects similar to those which 

 govern the measurement of distances upon the earth ; nor are they 

 attended with a greater difficulty, or more extensive sources of error. 

 By such means of observation and calculation it has been 

 ascertained that the sun's distance from the earth is a little less 

 than an hundred millions of miles. Although such calculations 

 supply results having surprising arithmetical accuracy, the ordinary 

 student will always find it convenient to register in his memory the 

 results in the nearest round numbers, and it is not easy to forget 

 that the distance of the sun, the most important of all astro- 

 nomical measurements, is very nearly an hundred millions of miles. 

 But while this round result is remembered, it will also be useful 

 to explain the more exact numerical measure of this distance, and 

 the limits of the error to which it is subject. The result of the 

 most exact observations made upon the different bearings of lines 

 drawn from opposite sides of the earth to the sun, gives as the 

 distance of that luminary, 



95,293452 miles, 



and it has been proved that this result cannot be greater or less 

 than the true distance by more than its three hundredth part. 

 We are, therefore, enabled to affirm absolutely that the distance 

 of the sun cannot be greater than 



95,293452+317645 = 95,611097 miles; 

 or less than 



95,293452317645 = 94,975807 miles. 



H2 99 



