CHAP. I.] PROTRACTORS 35 



tractors, which have extended arms, generally of very light 

 construction, which a slight blow will bend out of the direct 

 Hne. These sometimes admit of correction by means of screws, 

 which is easily accompHshed by placing the protractor on the 

 testing-sheet, with the opposite verniers exactly coinciding with 

 the same line, and adjusting the extending points until they 

 also prick precisely on the line. 



If the divisions of an ordinary protractor are found to be 

 incorrect, there is of course nothing for it but either to return 

 it to the maker to be re-cut, or to mark the errors at each ten 

 degrees, or wherever necessary, on small bits of paper pasted 

 on the protractor. 



A boxwood or vulcanite protractor is easily kept clean by 

 rubbing it over with a piece of india-rubber, but a brass or 

 electro-plated one is very apt to dirty the paper in plotting. 

 It is a good plan to carefully paste a piece of tracing-paper 

 on the under-side of these, when a rub with the india-rubber 

 before use will ensure cleanliness. The thinness of the 

 tracing-paper will not interfere with correctness in laying 

 off the angles. 



Vulcanite protractors are admirable for field work, as they 

 are light, easily read, and when made thick do not chip like 

 boxwood ones. 



Large brass circular protractors of 10 inches or so radius 

 are very useful for laying off the secondary points of a survey, 

 saving the time involved by using chords. 



POCKET ANEROID BAROMETERS. 



These are very useful when putting in the topography of a 

 country, as they give sufficiently accurate results for minor 

 heights, with but little loss of time ; but for the more exact 

 measurement of conspicuous hills, etc., they are but of little 

 use, and the theodolite and sextant must be had recourse to. 



In choosing a pocket barometer for the above-named service, 

 therefore, it is not necessary that it should read very low, as 

 it will be but rarely that nautical surveyors have to deal with 

 the intricacies of land over a few thousand feet high. For this 

 purpose, 25 inches is quite low enough, and the 5 inches of 

 barometric range thus obtained can be so largely marked on 



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