58 



(4.) The arc distance from the equator to the vertical is the Latitude of the ob- 

 server, whence the distance from the vertical to the polar axis, is the Co- Latitude. 



It will be observed that these arcs occur alternately around the entire circle; so 

 that the student should make himself familiar with their relative position with regard 

 to the horizon, and the vertical, in order to avoid mistakes, when setting the polar 

 of the instrument up to the pole of the equator. 



(5.) Astronomical Triangle. The height of the sun is measured in a plane passing 

 through the " Vertical " and the sun, and is called his Altitude, whence his distance 

 from the " Vertical " is his Co- Altitude. 



In the same manner, the distance from the sun to the "Pole," is his co-dec- 

 lination; and the distance from the "Vertical "to the pole, is the observer's Co- 

 LGtitude. These three compliments form what is called the Astronomical Triangle. 



Thus we have the three sides of a spherical triangle, from which to find the 

 several angles. 



(6.) The angle at the Pole, contained between the meridian of the observer and 

 that passing through the sun, is called the Hour Angle, as it gives the distance from 

 the sun to the observer's meridian, in time or arc, and is usually represented by the 

 letter H. 



(7.) The angle at the "Vertical," or at the observer's zenith, contained between 

 the meridian and a vertical plane passing through the sun, is called the Azimuth 

 Angle, and is usually represented by the letter Z. 



This angle is the one particularly important to surveyors, as from it the place of 

 the meridian is readily determined. 



Navigator's look for this angle every day, when an observation can be had, and 

 solve the triangle for Z, by one or both of the following equations. 



in which 



L = Latitude. Z = the required Azimuth 



d = Declination. p = Polar Distance = 90 d. 



h = Height of the sun's center, corrected for refraction and parallax. 



NOTE. The correction for parallax, which is usually about 6", and never exceeds 9", may be neglected 

 except in work of great precision. 



To solve these equations numerically requires much computation, but the Solar 

 Transit solves them for Z, mechanically, with no more computation than that required 

 to deduce the declination for the longitude and local time of the observer, from that 

 given iii the Nautical Almanac for the day. 



From the above definitions, it is readily seen that the following conditions, or 

 relation between the parts of the instrument, must be established. 



(A.) The polar axis must be Vertical, when the vertical arc (latitude arc) reads 

 zero, and, consequently, perpendicular to the cross axis of the transit telescope. 



(B.) The horizontal cross-wire of the solar telescope must be parallel with the 

 plane of its rotation around the polar axis; i.e. it must be parallel with the plane 

 of the equator. 



((7.) The plane passing through the vertical wire and the optical axis of the 

 solar telescope must be at right angles to the cross axis of the solar telescope. 



(D.) The bubble of the level-tube on the solar telescope must be in the middle 

 of its tube, when the optical axis of that telescope is in the plane of the horizon. 



These conditions are obtained by the following 



Adjustments. 



.ing attached the " Solar" to the cross axis of the telescope, as directed under 

 the head it" Remarka,'" 1 and having leveled up the transit (supposed to be in }n 

 adjustment! carefully, set the vertical or latitude arc to zero, observing that, upon 

 rotating tin-, \\hole instrument 180 in a/iniuth, (he bubble of the level of tin- transit 

 telesc' iie middle of the tube. Brinu r the level bubble of the solar telescope to 



the middle of the tube by means of the clamp and opposing tangent screws of the 



