Altitudes at Sea when the Horizon is invisible. 97 



field ; the inquired altitude of the lower limb of that object, 

 affected only by refraction, will then be found, as usual, on 

 the limb of the sextant. 



To enable the observer to keep the eye end of the bubble 

 aty till the required contact is observed, a light mahogany 

 rod, about %\ feet in length, attached to the sextant parallel 

 to the telescope, is pressed against some fixed object on deck, 

 which enables him gently to elevate or depress the telescope 

 till the bubble is brought into the required position, and kept 

 there as long as may be necessary. For this purpose, an iron 

 staunchion, about six feet in length, should be made to screw, 

 when required, into different parts of the deck near midship, 

 with a sliding projection, about two or three inches in length, 

 which may be fixed by a finger-screw at any required height, 

 so as to afford a convenient prop, against which the sextant 

 rod may be pressed by the observer, when taking observa- 

 tions. To show the cross hairs, and the position of the bub- 

 ble, when taking night observations, a small lamp, made for 

 this purpose, is applied to the right side of the eye tube by 

 means of a brass rod, fixed to the lamp, which slides in a 

 square socket, attached to the cylinder on the right of the 

 holder of the telescope. The quantity of light thrown upon 

 the bubble, and cross hairs, is easily increased or diminished 

 by moving the lamp rod a little forward or backward in the 

 socket. The same lamp, when detached, enables the observ- 

 er to read off his observations. 



The screw om, acting through the near end C of the level 

 frame, gives it its vertical adjustment ; and the two screws at 

 p, acting horizontally against each other through the farther 

 end D, give it its lateral adjustment. The accuracy of the 

 vertical adjustment may be examined by comparing meridian 

 altitudes of a celestial object, taken by means of the level, 

 with those taken at, or nearly at, the same time, by means 

 of an artificial horizon. At sea, the accuracy of this adjust- 

 ment may be examined by moving the index backwards, off 

 the limb, as many minutes as are equal to the dip of the ho- 

 rizon, and then observing whether the reflected horizon of the 

 sea is brought up to the horizontal hair in the centre of the 

 field, when the eye end of the bubble is at /in the level tupej 



VOL. IV. HO. i. tan. 1826. o 



