80 BEXJ. PIIiE's, JR., DESCRIPTIVE CATALOGUE 



as one continued arc of a circle ; on the contrary, when tho 

 arc appears broken, where the reflected and direct parts of 

 the limb meet, it is a proof that it wants to be rectified. 

 The second is, to make the horizon-glass perpendicular. 

 This is performed by a capstan-screw, at the lower end of 

 the frame of that glass ; and is known to be right, when, 

 by a sweep of the index, the reflected image of any object 

 will pass exactly over, or cover the image of that object 

 seen directly. The third adjustment is, for making the line 

 of collimation parallel to the plane of the circle. This is 

 performed by two small screws, which also fasten the collar 

 into which the telescope screws to the upright stem on which 

 it is mounted : this is known to be right, when the sun and 

 moon, having a distance of one himcLed and thirty degrees, 

 or more, their limbs are brought in contact, just at the out- 

 side of that wire which is next to the circle ; and then, 

 examining if it be the same, just at the outside of the other 

 wire : its being so is the proof of adjustment. 



Piioe $150 to $200. 



Theodolite. (Fig. 99, next page.) As an angular instru- 

 ment, the theodolite has from time to time received such 

 improvements that it may now be considered as the most 

 valuable instrument employed in surveying. Instruments 

 of this kind, of the best construction, may to a certain ex- 

 tent be used as altitude and azimuth instruments ; and seve- 

 ral astronomical operations, such as those required for de- 

 termining the time, the latitude of place, &c., may be per- 

 formed by them, and to a degree of accuracy sufficient for 

 most of the purposes that occur in the ordinary practice of 

 a surveyor. 



There are various modes of constructing theodolites to 

 suit the convenience or the views of purchasers ; but we 

 shall confine ourselves to a description of one of the most 

 perfect, as a person acquainted with the details of its adjust- 

 ments and use, will find no difficulty in comprehending those 

 of others. 



Description of the Theodolite. This instrument consists of 

 two circular plates, A and B, called the horizontal limb, the 

 upper or vernier plate, A, turning freely upon the lower, and 

 both have a horizontal motion by means of the vertical axis, 

 C ; this axis consists of two parts, external and internal, the 

 former secured to the graduated limb, B, and the latter to 



