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THE GUIDE T( ) NATURE 



The plunger, similarly cast, goes to 



a lathe where it is smoothed on the 

 outside. The crank is then added to 

 the piston, and held firmly in position 

 by clamps, bolts and nuts. Steadiness 

 of motion is given to the revolutions 

 of the crank by a lua\ \ balance wheel. 



Another ingenious device, known as 

 piston rings, is around the head of the 

 piston where it is intended to prevent 

 the escape of gas from the chamber of 

 the cylinder in which it is exploded; 

 that is, so that all of this force must be 

 used to move the piston. 



The revolutions of the crank are ap- 

 plied to a rod or shaft, on the end of 

 which is a wheel with hlades that push 

 the boat forward as the wheel rotates 

 in the water. 



Each revolution of the My or balance 

 wheel makes a break in the electric 

 current that causes a spark ; the electric 

 spark explodes the gasoline vapor over 

 the piston ; the force of the explosion 

 moves the piston ; the piston transfers 

 the movement to the crank ; the crank 

 carries it to the rod or shaft connected 

 with the propeller wdieel in the water. 



For aid in the preparation of this 

 article, the writer is indebted to Palmer 

 Brothers, Cos Cob, Connecticut. 



The Sextant. 



BY MORRIS W. EEE, HOBOKEN, N. J. 



The problem of the explorer in trac- 

 ing- his way over a barren and unchart- 

 ed territory is to continually pursue 

 the most direct path to his desired des- 

 tination, and thus conserve his energy 

 and supplies. On account of the ab- 

 sence of familiar landmarks or other 

 form of information, he must rely upon 

 the relative position of various heav- 

 enly bodies to the earth in order to 

 determine his latitude. This relative 

 position may be determined by obser- 

 vations with the sextant. 



The method usually employed is to 

 take an observation upon the sun at 

 noon, which is the moment it reaches 

 the greatest elevation above the hori- 

 zon. On land, such an observation may 

 De made by means of a fine engineer's 

 transit or theodolite, but at sea a sex- 

 tant must be used because of the ship's 

 motion. The adaptability of the sex- 



tant for use on land or sea makes it 

 specially valuable for explorers. 



The sextant is so named because the 

 graduated arc of the main frame is one- 

 sixth part of a circle. It is a reflect- 

 ing instrument, depending for its use 

 upon the following optical principle: 

 "When a ray of light has undergone 

 two reflections in the same plane, the 

 angle between the first and last direc- 

 tion of the ray is equal to twice the 

 inclination of the reflecting surfaces 

 to each other." This, while it may 

 seem a little complicated, is in reality 

 a very simple principle, depending 

 upon the well-known fact that a ray 

 of light leaves a mirror at the same 

 angle that it strikes its reflecting sur- 

 face. This is ordinarily stated, ''the 

 angle of incidence equals the angle oi 

 reflection." 



The sextant consists briefly of a 

 main frame carrying the graduated arc, 

 a movable index arm with the index 

 mirror at its pivot, a horizon mirror 

 on the main frame and a telescope 

 through which the sights are taken. 

 In addition to these main features there 

 is a handle suitably placed for holding 

 the instrument, a vernier with magni- 

 fying glass for reading the arc very 

 exactly, a slow motion screw for mov- 

 ing the index arm, and two series of 

 colored glasses to be placed between 

 the observer's eye and the sun. The 

 horizon mirror is silvered on one-half 

 of its surface only, so that in looking 

 toward it through the telescope one 

 sees both the object which is in the 

 direct line of sight and the reflection 

 of some object which is at another 

 angle. 



To measure the angle between the 

 sun and the horizon at sea or on level 

 sea ice, the observer holds the sextant 

 vertically in his right hand, looks 

 through the telescope at the horizon 

 through the clear part of the horizon 

 mirror and by swinging the index arm 

 about its pivot, brings the reflected 

 image of the sun so that its edge ap- 

 pears to touch the horizon line. The 

 reason for using the edge of the sun's 

 image is that a more accurate meas- 

 urement can thus be obtained than by 

 trying to judge its center. Such an 



