326 



Lieut. -Col. A. Strange on a new Theodolite [May 2, 



to two different tangent-screws having been accidentally made to act in 

 directions opposite to each other, the observation was disturbed in oppo- 

 site ways. 



In the present instrument I have adopted an arrangement which, I be- 

 lieve, is not new in principle, namely the " divided-nut" principle, some- 

 times used in machinery to prevent loss of motion. The block into which 

 the tangent-screw is tapped is divided transversely,' and the two halves 

 are forced asunder, and therefore against the contrary sides of the screw- 

 threads, by four internal spiral springs. The tension of these springs is 

 necessarily constant, and therefore not subject to the disturbance and slow 

 recovery of elastic force unavoidable in an external spring. Means are 

 supplied for regulating the tension of the four springs, which must be a 

 little in excess of the force necessary to move the revolving mass, without 

 taking the parts to pieces. This arrangement I believe to be on the whole 

 satisfactory. 



Bubble- trier.-— Experiments have constantly to be made to determine 

 the angular value of the scales of levels of such instruments. Hitherto 

 there have been no conveniences for this purpose. I have supplied a frame 

 attachable at will to the telescope, on which the level under experiment can 

 be laid, in order to compare its scale with the angular indications of the 

 vertical circle. 



Bearing of the Transit- Axis pivots. — Hitherto these have always been 

 capable of adjustment in order to level the axis: but to this two objec- 

 tions exist : — first, there is always ground for fearing lest the adjustable 

 parts should be left loose, in which case, owing to their distance from the 

 centre of motion, they would be disturbed by the momentum generated in 

 rotating the instrument in azimuth (since this would alter the relation 

 between the telescope and the horizontal circle, observed angles would be 

 vitiated to the extent of such disturbance) ; secondly, adjustable bearings 

 must necessarily support the pivots at points and not surfaces, and con- 

 sequently tend to wear them into grooves. 



For these reasons the bearings are immovably fixed and finally adjusted 

 by the maker by grinding. They are also of what is known as the seg- 

 mental form, first introduced, I believe, by the Astronomer Royal when 

 designing the great Greenwich Transit-Circle, a form which supports the 

 pivot throughout its length and over a considerable surface, and which has 

 been found at Greenwich to wear the pivots so equally that no sensible 

 change in their form in the course of many years has been detected. Still 

 the application of the non-adjusting principle to a large portable instrument 

 must be considered as an experiment. Should the horizontality of the 

 transit-axis undergo, as it may, a gradual change, the officers of the Survey 

 will have to restore the adjustment by means of the grinder supplied with 

 the instrument, an operation, no doubt, of some delicacy, even in the 

 hands of an experienced mechanic, but which, I trust, nothing but acci- 

 dental violence to the parts will ever render necessary. 



