SURVEYING INSTRUMENTS. 79 



degrees, neither the distance of the sun and moon, nor an 

 altitude of the sun, with the sea horizon, can be taken back- 

 wards ; because the dark glasses at that angle prevent the 

 reflected rays of light from falling on the index-glass; 

 whence it becomes necessary, when the angle to be taken is 

 quite unknown, to observe forwards first, where the whole 

 range is without interruption ; whereas, in that backwards, 

 you will lose sight of the reflected image about that angle. 

 But in such distances, where the sun is out of the question, 

 and when his altitude is taken with an artificial horizon (the 

 shade being applied to the end of the telescope), that angle 

 may be measured nearly as well as any other ; for the rays 

 incident on the index-glass will pass through the transparent 

 half of the horizon-glass, without much diminution of their 

 brightness. 



The advantages of this instrument, when compared with 

 the sextant, are chiefly these : the observations for finding 

 the index-error are rendered useless, all knowledge of that 

 being put out of the question, by observing both forwards 

 and backwards. By the same means the errors of the dark 

 glasses are also corrected ; for, if they increase the angle 

 one way, they must diminish it the other way by the same 

 quantity. This also perfectly corrects the errors of the 

 horizon-glass, and those of the index-glass very nearly. 

 But what is of still more consequence, the error of the cen- 

 tre is perfectly corrected, by reading the three branches of 

 the index ; while this property, combined with that of ob- 

 serving both ways, probably reduces the errors of dividing 

 to one-sixth part of their simple value. Moreover, angles 

 may be measured as far as one hundred and fifty degrees, 

 consequently the sun's double altitude may be observed 

 when his distance from the zenith is not less than fifteen 

 degrees ; at which altitude, the head of the observer begins 

 to intercept the rays of light incident on the artificial horizon ; 

 and, of course, if a greater angle could be measured, it 

 would be of no use in this respect. 



This instrument, in common with the sextant, requires 

 three adjustments. First, the index-glass perpendicular to 

 the plane of the circle. This being done by the maker, and 

 not liable to alter, has no direct means applied to the pur- 

 pose ; it is known to be right, when, by looking into the 

 index- glass, you see that part of the limb which is next to 

 you, reflected in contact with the opposite side of the limb, 



