428 Introduction to the Sixth Section 



shaft of that description which is expected from the great 

 artist himself. That description will not only detail the ori- 

 ginal course which this ingenious artist has pursued, but like- 

 wise contain the reasons for the whole and every part of the 

 instrument. 



The instrument is fixed between two pillars like a transit; 

 its horizontal axis, of 32 inches (Paris) length, terminates in 

 steel cylinders which rest on Y's of bell metal ; the latter are 

 formed of two planes meeting at an angle of 60°. Their co- 

 vers have springs, by which means, when the instrument is 

 perfectly balanced, the axis is pressed equally against the three 

 sides of an equilateral triangle. The equilibrium is pro- 

 duced by levers, the supports of which are on the columns 

 that stand on the pillars ; they carry the instrument by means 

 of friction wheels, which give to the whole an exceedingly gen- 

 tle and easy motion. The horizontally of the axis is ob- 

 tained by a fine level, the construction of which is such as to 

 ensure the greatest accuracy in levelling. 



On this axis are screwed the two halves of the telescope of 

 five feet in length, and secured against flexure by a lever ap- 

 paratus; the object-glass has 48*2 lines aperture; the four 

 eye-glasses magnify 66,107,129,182 times, and in the focus 

 there are five vertical and two horizontal wires, the latter be- 

 ing only 8'' distant from one another, which interval the ob- 

 ject that is to be observed bisects. Thus far the instrument 

 is, therefore, a complete transit, with the advantage that it 

 can be used with great ease in both positions of the axis. The 

 reversing of the axis, which must be done without the weight 

 of the instrument resting on the pivots, is obtained by a con- 

 trivance equally simple and secure, by which the counter- 

 poises begin to acts before the pivots reach the Y's. On the 

 one end of the axis there is fixed a cast circle of three feet 

 in diameter, which on the side towards the pillar is divided to 

 three minutes on silver. An alhidade circle is applied to the 

 same end of the axis in such a manner that the axis passes 

 through its centre and turns in a conical collar. This alhi- 

 dade circle has four verniers in a plane with the principal 

 circle, by which the latter is divided to 2" ; it carries a fixed 

 level with a scale divided to lines (Paris), by which the changes 

 of its position with regard to the horizon can be measured. 

 By a very strong arm issuing from its centre, and an adjusting 

 screw, it is attached to the pillar, so that the level would al- 

 ways retain the same position, were it not for the small changes 

 caused by the revolving of the axis of the instrument which 

 passes through the alhidade circle, by the temperature, or by 

 an alteration of the position of the pillars; the real existence 



of 



