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 
