526 Self-Calculating Sextant. 



bring all problems, as far as may be, under the influence of 

 this most simple rule. 



Were the uncommon accuracy in detail, and facility in 

 practice attainable by this instrument generally known, we 

 think that it would supersede many of the contrivances at 

 present in use for purposes of surveying. For example, it 

 would prevent the necessity of dragging a chain for the 

 measurement of distances, thus removing at once the most 

 laborious, unpleasant, and painful duty connected with the 

 profession. 



To measure a distance not exceeding four chains, nothing 

 more is wanted than a pole, with cross projections at top 

 and bottom, ten links apart. Erect the pole where you 

 please ; by a single observation (making the projections at 

 top and bottom coincide) you may at once tell your distance 

 in chains and links. Or sending a bearer off in any di- 

 rection, with the pole erect, you may tell him to stop at 

 any predetermined distance, ascertained by the coincidence 

 of the crosses. 



For greater distances (even of miles) similar means are 

 resorted to, on the ground plane. From one extremity of 

 the line to be measured, set off a perpendicular (which the 

 instrument readily enables you to do), measure on this line 

 a convenient distance, not less than one-fortieth part of the 

 whole distance to be measured ; then proceed to the other 

 extremity of the original line, and using the instrument 

 horizontally, bring the two former stations into contact. By 

 a single inspection you ascertain the distance required. 



The economy of time and labour, which this method 

 appears likely to effect, is obvious; it is independent of 

 temperature, seasons, crops on the ground, intervening ob- 

 stacles, and all the contingencies to which measurements 

 by the chain are liable. But what is more deserving of 

 notice is, that its accuracy seems to be superior to that of 



