Observations on Surveying Instruments. 271 



S, a screw acting against the spring, i, by which the com- 

 pass may be made fast to the stem of the ball. This part of 

 the compass which fits on to the stem, runs up through the 

 plates, and has attached to it in the face of the compass, an 

 index with a nonius, (see fig. 2.) If the screw at R, which 

 fastens the compass to the part just described, be loosed, and 

 the screw S made tight ; the compass may be turned around 

 at pleasure, while the index remains stationary — and thus 

 any contained angle may be measured, without reference to 

 the needle, with great facility. 



T and V, are sight-vanes, with eye-holes and cross-hairs for 

 levelling. They need be but short, as they can be elevated 

 or depressed to the direction of the object at pleasure. 



Fig. 2 shows an oblique view of the face of the compass, 

 with the needle, index, and levels, and also shows the manner 

 in which the parts of the joints are put together. 



This instrument, after a fair trial in surveying upwards of 

 five hundred miles, has fully answered my expectations. By 

 the addition of a telescope, which may easily be made to the 

 sight-vanes, it will unite all the excellencies of the theodolite, 

 and have the advantage, that it may be divested of its appen- 

 dages for measuring vertical angles, and rendered as porta- 

 ble as a common circumferentor. The trigonometer which 

 is attached to it, may be carried in the pocket, and if well 

 made, will entirely supersede the necessity of carrying a tra- 

 verse-table ; as on it, the latitude and departure for any 

 course, and any ordinary distance may be seen at a glance. 

 The surveyor will find it a very convenient and expeditious 

 method of measuring the distance across streams, &c. — par- 

 ticularly if the weather be wet and he cannot use tables. 



The experience that I have had with needles of different 

 forms, seems to confirm the truth of Professor Eaton's re- 

 mark, (Am. Jour, of Science, vol. XII, page 16,) namely, 

 that " the flat kinds are the best, which are wide in the mid- 

 dle, and of a true taper to the points. 1 ' I have been so well 

 convinced of this, that for several years I have used no other. 

 They should be nicely pointed, the south part blued by a 

 gentle heat, and the north part well polished. No letters, as 

 is usually seen in the face of compasses, to denote the cardin- 

 al points, ought ever to be placed on the card. They an- 

 swer a good purpose on the mariner's compass, where the 

 .position of the card is always the same with respect to the 

 cardinal points ; but on the surveyors compass, they will be* 



