146 SOME REFINEMENTS OF MECHANICAL SCIENCE. 



liable cargoes, the methods and instruments used in navigation have 

 been so improved as to greatly diminish the dangers in crossing the 

 seas. 



The perfection attained in the measurement of time, which is of 

 such great practical value in nearly every sphere of life, would not 

 have been possible were it not for the even greater refinements that 

 have characterized the methods and instruments used by the astrono- 

 mer in determining the length of the day and of the year, which are 

 the fundamental standards of time. 



The division of the circle and the measurement of angles have 

 ever been among the unsolved problems of the astronomer, yet in the 

 instruments used by him circles have formed a most important part. 



Long Ijefore the telescope was invented, Tycho Brahe, the Danish 

 astronomer, " the founder of modern astronomy,'' constructed for his 

 observatory instruments of various kinds having graduated circles 

 and arcs of circles. His instruments for the most part were improve- 

 ments on those used by Arabian astronomers in the eighth and ninth 

 centuries, and these in turn were copied after similar instruments used 

 by the (ireeks and Egyptians a thousand years previous, and it is sup- 

 posed that such instruments were used by the Chinese at an even 

 earlier period, so that graduated circles have come down to us from 

 the far-off ages. 



The longer the radius the more accurate the graduations, was the 

 principle upon which the early instruments were made. The Ara- 

 bians in about the year 1000 built a sextant with a 60-foot radius 

 and a quadrant with a il-foot radius, but to Tycho Brahe is due the 

 credit of constructing instruments having circles much smaller in 

 diameter and graduated with a greater precision than ever before. 

 It was by the use of such improved instruments of his own making, 

 and by his observations which were made without a telescope or any 

 means of magnification, that he was able to give the positions of a 

 large number of stars within less than one minute of arc from the 

 positions given by modern astronomers. 



The graduation of an 8-foot mural circle in 1725 by Graham, of 

 England, for the National Observatory, and of an 8-foot quadrant 

 by Bird, in 1767, were notable steps in advance in the division of the 

 circle and the measurement of angles; but these and similar instru- 

 ments, although their efficiency Avas greatly augumented by the use of 

 the telescope, have been supplanted by others more practical. 



The first circular dividing engine was made in 1710 by Henry Hind- 

 ley, of York, England, for cutting the teeth of clock wheels, and it is 

 interesting to note that in the same year Huntsmann, another clock- 

 maker, of Sheffield, invented the process of making crucible steel, 

 that he might have a metal suitable for the springs of his clocks. 



