THE EQUATORIAL.] 



ASTRONOMY. 



999 



or jet of gas. The shaded part (Fig. 201) represents a dia- 

 phragm with the cross wires, m, the angle of which may, 

 by turning the micrometer screw, a, be bisected by any 

 line on the circle in the field of view. On the left hand 

 of the diaphragm is a comb, or scale, each tooth of which 

 represents one minute ; and one revolution of the screw- 

 moving the wires over one tooth of the comb is equal to 

 one minute of space. 



The adjustment of the microscope consists in making 

 the cross wires in its focus, and the divisions on the 

 circle, both appear at the same instant of time, and free 

 from parallax : the adjustment is such that five revo- 

 lutions of the micrometer screw shall measure a five- 

 minute space on the graduated circle. For the former 

 of these adjustments in the telescope, the eye-piece is 

 drawn out until the distinct vision of the wires is ob- 

 tained, and the divisions or bars of the instrument are 

 well defined. 



The motion of the comb, or scale of minutes, is regu- 

 lated by a screw ; and the micrometer head, by friction, 

 can be made to read either zero, or any required second 

 when the cross wires bisect any particular bar, by hold- 

 ing the milled head of the micrometer screw. 



THE ECJI'ATOKIAL. The instruments we now come to 

 descril* belong exclusively to observatories, and in these 

 establish men ts the equatorial is specially adapted to 

 mark the diurnal movements of the heavens. The axis 

 A A (Fig. 202), round which the whole of the instrument 



Fig. 202. 



cun turn, ia so arranged as to follow the axis of our planet 

 tli.- r;irth. and it carries, laterally, the graduated scale 

 B B, which can turn both in its piano and round its 



centre. The telescope C C, fixed to this circle, follows 

 its movements, and its optical axis thus makes an angle 

 variable with the earth's axis. A second graduated 

 circle D D, whose plane is parallel to the celestial equator, 

 and having the axis A A as its centre, is fixed in such a 

 manner, that it follows all the movements of the instru- 

 ment in its rotations round this axis ; the position of this 

 second circle determines the altitudes, and gives uame 

 to the instrument. The clamp E E, with guiding and 

 reversing screws, intended to fix the circle and the tele- 

 scope to the axis A A, when the circle turns round its 

 centre, is carried by the pieces F F attached to the axis. 

 Two micrometers, G G, are adapted to the extremities of 

 the cross beam, firmly attached to the axis in such a 

 manner as to permit of the divisions of the graduated 

 scale, carried on the side of the limb of the circle, being 

 read off. Other micrometers are attached to the mason- 

 work which carries the extremities of the axis, and are 

 intended to read the graduated scale of the second circle 

 D D, which, in this instance, is on the upper face, and not 

 on the outside of the limb. Such an instrument affords 

 great advantages in measuring the relative position of 

 two contiguous bodies, in measuring the diameters of the 

 planets, <fcc. The circle which is connected with the 

 polar axis is graduated into hours, minutes, and seconds 

 of time, to indicate the right of ascension of the object 

 under examination ; while the circle connected with 

 the declination axis is graduated into degrees, minutes, 

 and seconds of arc, to indicate declination, or 

 polar distance. (See, also, the folio plate). 



From this disposition of the instrument it will 

 be seen that the optical axis or line of collima- 

 tion of the telescope turns towards all points 

 of the heavens, and, in making it turn with the 

 circle B B round its centre, it can be mode to 

 form any angle with the axis of the earth. If 

 the circle B B be fixed in one particular position, 

 by means of the clamp E E, and an entire turn 

 be made all round the axis A A, it is evident 

 that the optical axis of the telescope meets the 

 celestial sphere at various points of the sauio 

 parallel. 



A particular piece of mechanism, K, permits 

 the equatorial circle, D D, to be put in commu- 

 nication with a clock movement, so disposed that 

 the circle, D D, performs an entire turn on its 

 axis in a sidereal day. If the optical axis of the 

 telescope be directed to a star after the circle 

 B B is fixed to the axis A A, and the circle 

 D D put in communication with the clock-work, 

 the whole instrument will be carried along with 

 it, and the telescope will continue to follow the 

 movements of the star ; thus affording a means 

 of verifying the uniform rotatory movement of 

 the celestial sphere, after having made due al- 

 lowance for the effects of atmospheric refraction. 

 The telescope of the equatorial requires to be 

 in such a position that it can be directed to 

 every part of the heavens above the horizon. 

 This renders it necessary that the instrument 

 should be so placed as to be clear of interruption 

 from neighbouring objects. It is usual, there- 

 fore, to place it in the upper part of the observa- 

 tory. The following engraving represents a 

 section of the equatorial room of the Paris ob- 

 servatory, Fig. 203 representing a part of the 

 instrument, of which we shall here give a brief 

 description. 



The axis is supported at its lower extremity 

 by a massive piece of masonry, L, its upper 

 extremity being supported by the iron beam M, 

 made as light as the safety of the instrument, 

 and its required steadiness will permit. The in- 

 strument is protected from the weather by a 

 roof in the form of a hemisphere, having a long 

 opening following its vertical plane, and formed 

 with doors sliding laterally so as to leave a free opening iu 

 any required direction. By this arrangement the glass 

 can sweep the heavens on its vertical plane from the zenith 



