SEXTANT 



5329 



SHAD 



SEX'TANT, an instrument for measuring 

 the angular distance between any two points, 

 such as the sun and the horizon. It is used 

 most commonly by sailors in determining the 

 position of a ship at sea. 



A sextant con- 

 sists of a frame 

 supporting the 

 graduated arc of 

 a sixth part of a 

 circle, a radial 

 arm traveling 

 over this arc, two 

 mirrors and a 

 small telescope. 



The fixed mirror SEXTANT 



is known as the 



horizon glass, because it is trained on the hori- 

 zon; the movable mirror, screwed to the head 

 of the radial arm, or index bar, is called the 

 index glass. The purpose of the telescope is 

 simply to sharpen the line of the horizon. In 

 using the sextant, the operator, holding the 

 plane of the arc vertical, looks through the 

 telescope at the horizon glass, which is held 

 at the point where earth and sky seem to meet. 

 Then he moves the index arm until the image 

 of sun or a star, reflected in the index glass, 

 touches the horizon line. The sun's altitude 

 may be read from the graduated arc, slight cor- 

 rections being made for refraction and other 

 errors. 



The principle underlying the use of the sex- 

 tant is a rule in optics: If an object be seen 

 by repeated reflection from two mirrors perpen- 

 dicular to the same plane, the angular distance 

 of the object from its image is double the in- 

 clination of the mirrors. In reading the gradu- 

 ated arc, therefore, half degrees are taken as 

 degrees, because what is really measured on the 

 index is the angle between the mirrors, and this 

 is half the distance between the objects. The 

 position of the star in the celestial sphere being 

 known, it is possible to determine the latitude 

 of a ship by comparing its altitude with this 

 position. 



SHACKLETON, shak"lton, SIR ERNEST 

 (1874- ), an Antarctic explorer, born at Kil- 

 kee, Ireland. He was sent to Dulwich College, 

 but his love of the sea was stronger than his 

 love of books, and he left college without finish- 

 ing his course. Four times he sailed around 

 the world before his appointment, in 1901, as 

 third lieutenant of the National Antarctic Ex- 

 pedition. From 1904 to 1906 he was secretary 

 and treasurer of the Scottish Geographical So- 



ciety, and in the latter year made an unsuc- 

 cessful attempt to enter Parliament from 

 Dundee. 



In 1907 Shackleton was placed in command 

 of the British Antarctic Expedition, and landed 

 in 1908 at Erebus 

 Island, South Vic- 

 toria Land. Here 

 he spent the win- 

 ter, sending out 

 sledge parties of 

 exploration. 

 Mount Erebus 

 was ascended, 

 and an expedi- 

 tion, one of the 

 most remarkable 

 sledge exploits 

 ever recorded, 

 was undertaken S IR ERNEST SHACKLETON 

 over the Antarc- 

 tic continent. On January 9, 1909, the party 

 reached latitude 80 23', a point only 111 miles 

 from the South Pole. The elevation here was 

 11,600 feet, and the wind and storms were of 

 great violence; this fact, in conjunction with a 

 shortage in food supplies, compelled the party 

 to turn back when so near the goal. Shackle- 

 ton was knighted on his return to England and 

 was presented with numerous honors and 

 medals by societies in all parts of the world. 



Late in December, 1914, the explorer sailed 

 from South Georgia, in the South Atlantic, 

 beginning then an expedition that was destined 

 to be one of the most thrilling in the history 

 of polar research. The hardships and perils of 

 this journey are described in detail in these vol- 

 umes in the article POLAR EXPLORATION (page 

 4729). Shackleton returned to England in the 

 spring of 1917, to aid his country in fighting the 

 great war. He passed through the United 

 States on his homeward journey, lecturing there 

 to large and enthusiastic audiences. 



Consult Shackleton's Heart of the Antarctic. 



Related Subjects. The reader is referred to 

 the following articles in these volumes : 

 Amundsen, Roald Ross, James Clark 



Polar Exploration Scott. Robert Falcon 



SHAD, an important food fish of the herring 

 family, ranking in economic importance next to 

 the salmon and the cod among the fish of the 

 United States. The common, or American, 

 shad is found from Newfoundland to Florida 

 and has been introduced into the Pacific coast 

 waters by the Bureau of Fisheries. It is pri- 

 marily a sea fish, but ascends fresh-water rivers 



