m. MISCELLANEOUS. 621 



nifying 35 times, adapted for revolving and lying down, level for 

 setting ; tangent screw exactly 1 mm. range. 



A. and R. Hahn, Cassel. 



By means of this instrument elevations can be measured directly with 

 the tangent instead of by the arc or its chord as hitherto. Its advantage over 

 other screw instruments consists in its peculiar construction, which admits 

 of using the micrometer screw in the measurement of angles up to 45, whilst 

 till now angles to about 10 only have been so measured. The relation 

 between the reading on the screw and the height or distance to be measured 

 is a simple geometrical one. 



3051. Repetition Theodolite (No. 148), with stand and 

 leather case. Otto Fennel, Cassel. 



Horizontal circle with covered silver border 14 cm. in diameter, gra- 

 duated into one-third degrees, vernier to 20 seconds. Achromatic double 

 lenses, orthoscopic telescope. Vertical circle with vernier graduated to 

 minutes. 



3052. Metford's Improved Theodolite, made by F. Pasto- 

 relli, under the direction of the inventor. Francis Pastorelli. 



DESCRIPTION OF THE INSTRUMENT. 



Levelling Gear. It consists of three left-handed screws with the balls 

 fitting closely to their beds on the under surface of the traversing box ; they 

 are secured by an elastic three-cornered plate, having boxes at their ends to 

 protect them from dust and grit. The lower ends of the three screws pass 

 through a triangular plate with broad ends, with sufficient spring to permit 

 the female screws to be slightly twisted. 



Traversing Stage. The main hollow centre of the instrument carries a 

 circular disc 3| inches in diameter and - of an inch thick ; the traversing 

 stage is a flat plate 5^ inches in diameter ; in its bottom the levelling screws 

 are seated as previously explained. The upper surface has a ring round its 

 edge the depth of the circular disc. There is a Jg. of an inch hole in the 

 stage to let the plumb cord traverse with the instrument; the disc will 

 traverse 1 inch in any direction from the centre. To secure the instrument 

 there is an upper plate screwed to the ring, so that the stage becomes a very 

 shallow box ; there is also a washer that keeps out dust and grit. A three- 

 arm pinching screw running on the hollow centre secures the disc. 



Horizontal Limb and Vernier Plate. The horizontal limb and vernier plate 

 are solid ; the latter has mounted upon it a compass, circular bubble, and a 

 memorandum plate ; also attached to it is the tangent motion. The limb is 

 divided to read 20 minutes, the vernier to 30 seconds. 



Bracket Support of Vertical Limb and Telescope. On the side of the main 

 pivot is attached a strong curved bracket with two arms at the top. This 

 bracket has a T section throughout ; to it is fixed the vertical circle and 

 female centre. The telescope and vernier circle, with its tangent motions, 

 are fixed on the male centre. The main spirit bubble is fixed at the back of 

 the vertical circle ; there is a screw in the bracket for perfecting its adjust- 

 ment. The axis of the telescope is suspended over the axis of the instrument, 

 and admits of a transit motion ; this is an important improvement. 



Tripod Staff Head. This is an adoption of W. Froude, Esq., C.E., by 

 which steadiness is obtained, which is of great importance in taking angular 

 measurements. The checks are set wider apart, the leg joints being similar 



