Hassler''s Repeating Theodolite. 253 



Mr. Patten, if I omitted to bear testimony to the excellency 

 of his work, not only in the construction of this, but also of 

 other instruments for the expedition. The beauty and ac- 

 curacy of the division, (an operation hitherto deemed by 

 many impossible in this country,) would not sutler in com- 

 parison with that of European artists. He cannot be too 

 highly recommended to the patronage of the public. 

 Your obedient servant, 



Charles Wilkes, Jr. 

 Professor Silliman. Lieutenant U. S. Navy. 



Fig. 1 is a perspective view of the instrument. 

 ' Fig. 2. The horizontal circle with its alhidades, &c. &-c. 



Fig. 3. The vertical circle with its alhidades, &c. 



Fig. 4. A section of the whole instrument. 



The letters of the perspective view answer to those of 

 the sectional parts. 



«, a, a, are three horizontal radii of six inches radius, in 

 the ends of which are simple levelling screws, which fit in 

 sockets on similar radii, &, d, 6, screwed to the top of a three 

 legged stand, which serves as the support of the instrument 

 in the field, b, b, is the horizontal or azimuth circle, of nine 

 inches diameter, with a silver arch attached to the middle 

 conical center, y, divided and read oflT, by means of verniers, 

 to fifteen seconds, c, c, is the lower or standing alhidade, 

 attached to the outer centre, x, having four arms, three bear- 

 ing verniers, the fourth a clamping and tangent screw, for 

 slow motion to the circle, d, d, is the upper or moving alhi- 

 dade, attached to the inner conical centre, z, having four 

 arms, three of which bear verniers, the fourth a clamping and 

 tangent screw, for slow motion, on two of these arms are 

 placed the hollow conical pillars, e, e, of an inch and a quar- 

 ter diameter at their basis ; to the top of these is fixed the 

 Y's, for the axis of the telescope, having a vertical adjust- 

 ment in one of them. /,/, is the telescope, of twelve inches 

 in length, with its conical hollow arms, g,g ; the pillars, e, e, 

 are of sufficient height to allow the telescope to have a free 

 motion through the vertical ; on one of the arms is fitted the 

 vertical circle, h, h, of six inches diameter, with a silver arch 

 moving on a bell metal collar, and read oft" by means of ver- 

 niers to fifteen seconds, having two alhidades similar in every 

 respect to the horizontal circle, i, i, is the inner alhidade, 

 attached to and moving with the telescope, having four arms. 



