1 66 PHYSICAL GEOGRAPHY. 



(the time being noted), and the proper correction applied, the true 

 meridian will be determined. 



If a transit or theodolite is used, it must be carefully adjusted, or the 

 results will be less reliable than by the simple plumb-line, as above. The 

 adjustments liable to affect the work are the collimation, and the height 

 of Y's or horizontality of the axis of the telescope. By making two 

 observations, one with the telescope reversed, and using the mean 

 result, any error which would otherwise occur by defect in these adjust- 

 ments is eliminated ; but in this case both observations cannot be taken at 

 the moment of culmination : hence, for one, at least, the azimuth must be 

 computed. If neither is taken at the culmination, separate azimuths must 

 be computed for each. 



Unless an instrument with a perforated axis for illumination of spider 

 lines is used, some easily managed means must be contrived for this 

 illumination. If a steady light cannot be thrown upon the lines in such 

 way that it may be increased or diminished at pleasure, it is not easy to 

 see both the star and lines with that distinctness necessary to a good 

 observation. With the perforated axis there is little difficulty in securing 

 the right amount of light ; but without this, the light must be thrown 

 into the object end of the telescope. This can be clone successfully by 

 using a stand to carry a bull's-eye lantern, and a vertical piece of board 

 covered with white paper to serve as a reflector, the diffused, reflected 

 light being much better than the direct rays of the lantern ; or, a ring of 

 thin white paper, of suitable size to cover the outer edge of the object glass, 

 leaving the centre open, may be made to adhere to the glass by simple 

 wetting, which will serve to reflect and diffuse the light thrown upon it. 



To mark the meridian after the observation, a piece of board with a 

 small hole, behind which a light is placed, will serve as a temporary 

 arrangement. This need not be placed precisely in line of the meridian, 

 but being fastened, before observing, at some point near this line, the 

 angle between it and the star may be taken, and the work of fixing and 

 permanently marking the meridian be deferred to any convenient time. 

 When the final marking is done, it should be such that neither frost nor 

 any other natural causes will disturb it. 



2. A meridian may be established by observations upon the sun ; but, 

 while they offer the advantage of the day-time for doing the work, they 



