296 PHILOSOPHICAL TRANSACTIONS. [aNNO 1772. 



index till the sun or moon's limb, or the star touch the horizon. The nearer 

 the object is brought to an imaginary line in the middle between the wires, (it is 

 indifferent what part of the line it is brought to), and the truer the wires are kept 

 perpendicular to the horizon, the more exact will the observation be. In the 

 fore observation, the object appears in its real position ; but in the back observa- 

 tion, the object being brought through the zenith to the horizon, the real upper 

 limb will appear the lowest; and the contrary. Either limb of the sun may be 

 used in either observation ; but it will be most convenient in general to make 

 the sun appear against the sky, and not against the sea ; and then the objects 

 appearing itiverted through the telescope,- the sun will appear lowest, and the 

 horizon highest. The observed altitude is to be corrected for dip, refraction, 

 and the sun's semidiameter, as usual. 



In taking the distance of the nearest limbs of the sun and moon, whether by 

 the fore or back observation, having first set the index to the distance nearly, by 

 the help of the nautical almanac, and brought the moon to appear any where on 

 or near the diameter of the field of view of the telescope, which bisects the interval 

 between the wires, give a sweep with the quadrant, and the sun and moon will 

 pass by one another; if in this motion the nearest limbs, at their nearest 

 approach, just come to touch one another, without lapping over, on or near 

 any part of the diameter of the field of the telescope which bisects the interval 

 between the wires, the index is rightly set: but if the nearest limbs either do not 

 come to meet, or lap over one another, alter the index, and repeat the observa- 

 tion till the nearest limbs come to touch one another properly. This method of 

 observing will be found much more easy and expeditious than without the wires, 

 since in that case it would be necessary to make the limbs touch very near the 

 centre of the telescope, but here it is only necessary to make them do so any 

 where on or near the diameter of the field of the telescope which bisects the 

 interval between the two wires. The same method may be used in taking the 

 moon's distance from a fixed star. 



It may not be amiss here to make some remarks on the rules that have been 

 usually given for observing the sun's altitude, both with the. fore and back 

 observation, which have all been defective, and to point out the proper directions 

 to be followed, when a telescope is not used with two parallel wires to direct the 

 quadrant perpendicular to the horizon, and to show the principles on vvhich 

 these directions are founded. 



Observers are commonly told, that in making the fore observation, they 

 should move the index to bring the sun down to the part of the horizon directly 

 beneath him, and turn the quadrant about upon the axis of vision ; and when 

 the sun touches the horizon at the lowest part of the arch described by him, the 

 quadrant will show the altitude above the visible horizon. I allow that this rule 



