Oy DRAWING, WETTING, AND MEASURING. 105 



not easily carry the accuracy of our determination, although, in calculations, 

 we sometimes descend as far as tenths or even hundredths of a second. The 

 decimal division of a right angle, which has been lately adopted in France, 

 appears to have very little advantage for the purposes'^ of calculation, beyond 

 the common method, and its execution in practice must be much more 

 ditHcult. 



Whole circles, or theodolites, divided into degrees and their parts, quadrants 

 and sextants, are usually employed in measuring angles; and protractors, se- 

 micircles, and lines of chords, in laying them off. The most convenient of 

 quadrants for general use is Hadley's reflecting instrument, which is in fact 

 an octant or a sextant, but in which, for reasons depending on optical prin- 

 ciples, each degree of the arc is reckoned for twc 



For the graduation of all instruments of this kind, of moderate dimensionSy 

 Mr. Ramsden's dividing engine is of great utility ; the instrument being fixed 

 on the revolving plate of the engine, its arc is made to advance under the 

 cutting tool by very minute steps, regulated by the turns of a screw, of which 

 each revolution is divided into a considerable number of equal parts. The 

 largest and finest instruments are, however, still usually divided by hand, 

 that is, by means of compasses. Some artists have first divided a straight 

 plate, and then made a hoop of it, which has served as a standard for further 

 processes. An arc of J° 10', of which the chord is one eighth of the radius, 

 may be employed as a test of the accuracy of the work. A micrometer screw . 

 is often used in large instruments as a substitute for the minutest divisions; a 

 moveable part of the index being brought to coincide with the nearest point 

 marked in the arc, by turning the screw through a part of its revolution, 

 which is measured by means of a graduated circle. But a simpler method of 

 reading off divisions with accuracy in common instruments, is the application 

 of a vernier, an apparatus so called from its inventor. The space occupied 

 by eleven divisions of the scale being divided into ten parts on the index, the 

 coincidence of any of the divisions of the index with those of the scale,, 

 shows, by its distance from the end, the number of tenths that arc to be added 

 to that of the intire divisions. (Plate VII. Fig, 92.) 



There are several ways of measuring the angular elevation of an object 



YOL. I. B 



