THE TRANSIT 77 



when the line of sight is horizontal. Having made the 

 axis of the telescope bubble parallel to the line of sight, 

 bring the bubble into the center of the tube and adjust the 

 vernier of the vertical circle until it reads zero on the limb. 

 If the vernier is not adjustable, the reading in this position 

 is its index error, to be applied to all readings. 



2. CARE OF THE TRANSIT 



The transit should be protected from wet and dust as 

 much as possible, a waterproof bag to cover it being useful 

 for that purpose. The tripod legs should move freely, but 

 not too freely ; there should be no lost motion about their 

 shoes or elsewhere. Dust or water should be removed from 

 the glasses by a camel's hair brush or the gentle use of a 

 clean handkerchief; grease may be removed by alcohol. 

 Care should be taken not to strain the parts of the instru- 

 ment by too great pressure on the screws when using or 

 adjusting it. Before the transit is picked up, the levelling 

 screws should be brought approximately to their mid po- 

 sition, the telescope should be turned vertically and lightly 

 clamped, and the clamp of the lower plate should be loos- 

 ened. Then, if the instrument strikes anything while being 

 carried from point to point, some part will move easily and 

 severe shock will be avoided. 



3. STADIA MEASUREMENT 



Measurement of distance by stadia is secured by simply 

 sighting with a transit at a graduated rod held on any de- 

 sired point and noting the space on the rod included 

 between two special cross-hairs set in the focus of the in- 

 strument. This is a very rapid method of measurement, 

 being especially handy and effective over broken land; it 

 gives a degree of accuracy sufficient for very many pur- 

 poses; it allows the computation of the difference in ele- 

 vation between two points. Thus for many purposes it is 

 the most effective method of survey, and it is coming 

 into general use. 



The Instrument. A transit intended for stadia work is 



