of this instrument see Sir Jonas Moore's work, before referred 

 to, which also gives descriptions of the Demi Cross Bow 

 and Plough, which were all modifications of the cross staff.) 



Back Staff, Sea Quadrant or Davis's Quadrant. Called 

 Back Staff because the back of the observer was turned to 

 the sun or star when observing, and Davis's Quadrant from 

 its inventor, Captain John Davis, who discovered the strait 

 bearing his name and who died in 1605 (described and illus- 

 trated by John Seller in his " Practical Navigation," of 

 which there are several editions, 1672-1720). The instru- 

 ment consists of two graduated arcs, one of 30 connected 

 by radii to a centre, and a second of smaller radius measuring 

 60 on the side of it. To the 30 arc a sight vane is attached, 

 and on the 60 arc another for the purpose of carrying the 

 ray of the sun to the vertex of the instrument, where there 

 is a horizontal vane with a slit in it, through which the 

 horizon can be seen ; the altitude is read off by adding the 

 amounts shown on the two arcs. 



Hadley's Quadrant or Octant (this being the form of the 

 instrument) was invented by John Hadley in 1730, and was 

 the first reflecting instrument of any note that had been 

 made; it has been in general use since the time it was 

 invented, and, with very small modifications, is practically 

 of the same construction as the sextant in use to-day. The 

 original is thoroughly described by Benjamin Martin in his 

 " Theory of Hadley's Quadrant Demonstrated." The first 

 examples were made of mahogany, of 20 inches radius, with 

 the arc divided by a diagonal scale of boxwood, with a 

 wooden index bar; later examples were of mahogany or 

 ebony, with ivory scale, with a nonius or vernier reading to 

 one minute. The invention of this instrument was also 

 claimed for Newton, but the Royal Society examined the 

 claims and decided that the invention belonged to John 

 Hadley. As time went on it was found desirable to make a 

 more extended arc, and sextants reading to 120 were made, 

 and of smaller size, some of only 3 inches radius, for the use 

 of travellers. During the iSth century splendid specimens of 

 the sextant were made by J. Ramsden, B. Martin, P. Dollond, 

 G. Adams, J. Carey and others, which, as examples of hand- 

 work, have never been excelled, and which show that no care 

 or trouble was spared to produce the best possible instru- 

 ment. A specimen of Ramsden's Pillar Sextant, now in a 

 private collection, is divided to 5" ; the arcs were frequently 

 divided on gold and in platinum. There are many 

 varieties of sextants, among which may be mentioned the 



