## 66. If α is the observed altitude, the refraction correction in seconds, is

## 67. The elevation of the star at elongation is obtained by

## 68. For adjusting a quadrilateral whose both the diagonals are observed, the equations of conditions involved, are

## 69. An imaginary line lying throughout the surface of ground and preserving a constant inclination to the horizontal is known as

## 70. The angle of intersection of the two plane mirrors of an optical square is

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