350 SURVEY OP THE COAST 



Before the object end of the telescope there is a light 

 lengthening tube of three inches in length, to keep the side 

 light and glare from the object glass, winch is very necessary 

 in this instrument. 



The eye part of the telescope is in a long tube sliding in 

 the main tube. By sliding it in and out, the wires are placed 

 in the focus of the object glass ; and by turning it in the tube 

 they are placed perpendicular. 



There are three vertical and one horizontal wires. The 

 multiplication of the vertical wires is for the observations of 

 transits of celestial bodies in observing transits, by which a 

 survey may be properly oriented, or a true meridian drawn 

 in it. The eye pieces may be chosen at the pleasure of the 

 observer, and, in respect to their magnifying power, they must 

 be adapted to the middle distance of the objects which may 

 come under observation. It is therefore proper to liave se- 

 veral clianges ; and it will be very convenient to have one 

 prismatic, which being placed so as to look upwards, will 

 serve for objects at such an elevation as will not easily allow 

 room for the head between the plane of the table and the 

 eye piece ; and it would not be proper to give to the instru- 

 ment too great an elevation in its construction, because it 

 would affect the stability of the vertical plane of the tele- 

 scope. 



The spirit level k hangs to the axis by hooks, and has 

 proper adjustments. It serves for the levelling of (he axis 

 of the telescope, and the plane table itself. The method of 

 using it is too evident and simple to be detailed here. 



As this instrument will, in its use, be placed on diffcient 

 parts of the j)lane table, it becomes a desirable object that it 

 should be as light as may be consistent with its necessary 

 solidity, in order that it may not affect the level of tiie plane 

 table. For this purpose, tiie large rule is cut out as seen in 

 fig. 3, so as to form only a skeleton to tlie outei- straight 

 line, and the sujjports of the telescope. In like manner, the 

 frame J, d and the supports e, e are cut out, so that the in 



