106 Mr T. Stevenson on Levelling Instruments, 



operation which is done almost instantaneously.* The socket- 

 screw (N) is then clamped, and the telescope bubble (HH) is 

 brought to the absolute level by a slight touch of the parallel 

 plate-screws (D). In this way the legs of the tripod never need 

 to be moved after the instrument has been placed on the 

 ground, and the parallel plate-screws have almost nothing to 

 do — advantages which all who are accustomed to levelling will 

 fully appreciate. 



In levelling over mountainous districts, it very often hap- 

 pens that it is desirable to select a station where the ground 

 is so rugged and precipitous as to render it difficult, if not im- 

 possible, to find three points for the extremities of the legs of 

 the instrument to rest on, which shall be on such levels as to 

 bring the telescope within the range of the parallel plate - 

 screws ; but wherever the instrument can be made to stand with 

 safety i the bubble of the improved level can be adjusted, and 

 adjusted in exactly the same time^ and with exactly the same 

 ease, as if the instrument were placed on level ground. 



Another advantage of these improvements is the removal of 

 a great practical difficulty which is often experienced on slop- 

 ing ground. The instrument being set and properly adjusted, 

 the observer, on looking through the telescope, may discover 

 that he is not within the range of the levelling-staff ; in other 

 words, he has chosen a station too high or too low to admit of 

 his seeing any part of the staff within the field of the object- 

 glass. The only remedy for this is to choose a new station 

 where the instrument must be again set up and levelled, at a 

 great expense of time and trouble. In order to remedy this, 

 it was my intention at one time to have fixed on the telescope 

 a French level, on the principle of the plummet, in order 

 speedily to discover, before making the adjustments, whether 

 the intended station were within the range of the staff or not. 

 But the instrument can be roughly set with so much quick- 



* In the annexed plan the instrument is shewn off the level, so that 

 neither the air-bubble of the circular level (G) is in the centre of the 

 circle ; nor does the air-bubble in the tube (H H) correspond with the 

 file-marks made on the glass. 



