37 



The Centers. 



Quite as important as the graduation, is the exact fitting of what the makers 

 call the centers of the instrument ; i.e., the two vertical metal axis, about which (fhe 

 circle and the vernier plate turn. 



Both axes must be exactly concentric with the center of the graduated circle, 

 and the center of the horizontal axis of the telescope in any position of the instru- 

 ment. The most sensitive level about the instrument should not show any displace- 

 ment when the circle-plate is held, and the lower plate moved by the hand. 



In the construction of the inner center, the hardest bell-metal should be used, 

 and for the outer center a red composition metal of the best quality. To insure a 

 true concentricity of the axis, and consequently of the limb and vernier, it is 

 necessary that they should each be turned in a dead center lathe, each about its 

 own axis. In fitting the centers, they should turn without the slightest play, and 

 yet with very little friction. 



Messrs. C. L. Berger & Sons take the precaution of casting the outer center, circle 

 and vernier plate in the same mould, to avoid any difference in the composition of 

 the metal. 



The upper plate should not be hammered, since this would also effect an unequal 

 expansion of the metals in extreme temperatures, causing the vernier to read too 

 long or too short. 



After the plates are put together, the vernier and limb should revolve in the 

 same plane, to avoid parallax. The space between the limb and vernier should 

 have the appearance of a uniform, fine, black line. 



The Compass. 



In running old lines, and as a check in running new ones, the compass is frequent- 

 ly a very important part of the transit. Its needle should be tempered throughout, 

 and of hard steel, to retain its magnetism. It should be thin, and yot at the gams 

 time have enough surface to be strongly magnetic. It should bo swung upon a 

 jewelled center, and so nicely fitted that when at rest, with the instrument levelled, 

 the two extreme points should just clear the graduation of the compass box, and 

 read precisely 180 different in any part of the graduated arc. The pivot on which 

 it swings should be conical, and hardened so that it may swing upon a sharp point, 

 without having this point weak. 



The needle should also be so sensitive, that when drawn from its pointing by 

 the outside attraction of a piece of iron held in the hand a foot or so away, it will 

 settle to the same reading several times in succession. 



This sensitiveness depends upon the form and sharpness of the pivot, the strength 

 of its magnetism, and its bearing on the jewelled center. 



If it should be found that a needle has lost its sensitiveness, it is probably not 

 so much owing to its loss of magnetism, as to a dulling of the pivot. ^ Since this 

 may happen when the engineer is without access to the maker, and an. instrument 

 otherwise be in good condition, it should be remarked that the pivot can be sharp- 

 ened after removing the needle, by taking a fine oil-stone, and while turning the 

 instrument with one hand, grinding the pivot, with the oil-stone in the other ; being 

 careful to incline the grinding surface about 25 to the pivot. The pivot is origi- 

 nally turned and sharpened in a. lathe, and in grinding by hand, great care should 

 be taken to preserve its conical form. 



The two extreme points which lie next the graduation, together with the point 

 of suspension, should lie in one straight line. 



The center of gravity of the needle should be as far below this line as possible. 



The quivering of a needle so constructed is not annoying, since the center of its 

 quivering motion is in the line through its two extreme points, which are, therefore, 

 stationary. 



To determine whether the transit itself has any iron in it to disturb the needle, 

 it is a good plan, after setting the instrument so that both compass-needle and 

 vernier reads 0, to go round the circle, setting the vernier ten degrees ahead each 

 time, and noting whether the compass-needle also describes an arc of precisely ten 

 degrees. If it does not, there is some local attraction. 



