752 SEC. 15. GEOGRAPHT. 



higher magnifying power in the eyepiece, the advantages of a much larger 

 instrument are obtained. A mirror is placed over the bubble, so that ..the 

 operator can see, while reading the levelling staff, if the instrument keeps its 

 proper position ; this being necessary on soft and spongy ground. 



31O5a. 14-in. Dumpy Level, with improved rack adjust- 

 ment to cross webs. Henry Porter. 



31O5b. Twelve-inch Gun Metal Engineer's Level, with 

 channelled bottom, for setting flat work, shafting, &c. Very 

 delicate. Joseph Casartelli. 



3105c. Small Improved Builder's Level, working parts 

 all made of gun-metal. Joseph Casartelli. 



31O5d. Ten-inch Dumpy Level, with long centre im- 

 proved construction, and the working parts all made of gun-metal. 



Joseph Casartelli. 



3105e. 12-inch Level. G. W. Strawson. 



3105f. 10-inch Level. G. W. Strawson. 



3110. Level, with independent bubble, Gravet's system, with 

 stand. M. Tavernier Gravet, Paris. 



3112. Patent Surveying Level. Francis Pastorelli. 



This instrument combines several important improvements, including 

 increased facility in use, greater steadiness and freedom from vibration and 

 possibility of derangement, accurate adjustments, and great durability. 



Tripod and Staff Head. Stability is of the utmost importance. This is 

 secured in the staff head (which is the adoption of a plan by Win. Froude, 

 C.E., F.R.S.), which has the cheeks set wider apart, the leg joints being similar 

 to an inverted mortar with strong trunnions, which can be tightened in their 

 bisected cylindrical bearings by means of capstan headed screws. As wear 

 goes on they can always be kept perfect ; this cannot be done with the ordi- 

 nary staff head. 



Ball Joint and Clamp. The instrument is free to move with 20 of inclina- 

 tion. This is most important, as much valuable time is saved, more especially 

 upon hilly ground, as it can be almost set instantaneously. 



Adjustments. The parallel screws work in movable hemispherical nuts, 

 which are held in seats in the parallel plates, those in the lower being held in 

 brackets. Their action permits the upper plate to be worked at an inclination 

 of about 15 without their being strained or twisted. By their means the 

 upper parallel plate is made to clamp or set free the inverted cup on the 

 vertical axis of the instrument ; in addition, they cause it to heel over to per- 

 fect the adjustment, so as to bring the main bubble in the centre of its run 

 whenever a force is applied to them greater than is necessary to clamp the 

 inverted cup. 



Suspension of Telescope. The telescope is solidly fixed to its base, which 

 is parallel to the axis of it ; it has a female screw with a true surface which 

 fits on to the gun metal centre, so that when the instrument is reversed in any 

 direction the main and circular bubble is retained in the centre of their run. 

 Around the telescope are two gun-metal collars, accurately turned and ground 

 to a perfect circumference ; by their means are adjusted the mechanical and 



