REPORT OX A LEVEL LINE. 13 



ricd with us a piece of canvas, 6 feet square, nailed to two poles, 

 ^vhich were sharpened at the bottom, to enter the ground. This 

 screen being held firmly by two men on the windward side of 

 the instrument, sheltered it so completely, that T was able to 

 proceed in windy weather, with but little interruption. 



The brass leveling staff was employed in leveling between 

 Bristol and Portishead ; but being found inconvenient, and liable 

 to get out of repair, was obliged to be laid aside. The staff 

 which I subsequently constructed and used, is of wood, 9 feet 

 long, and 2 inches wide, a single piece of straight- grained oak. 

 On the face are two different scales of equal parts. One is the 

 common scale of feet and hundredths of a foot ; the other has 

 larger divisions, in the proportion of 19 to 16 nearly, or more 

 exactly, as 1*18702 to 1 : an aliquot ratio of the scales having 

 been purposely avoided. Both of these are reckoned upwards 

 from a common zero at the bottom of the staff. The centesimal 

 divisions of the foot are produced in strong black lines towards 

 the left, and large figures denoting feet and tenths placed against 

 them, so that the height may be read off at the telescope to the 

 j-igth part of a foot at a distance of 150 or 200 yards. These 

 marks are also useful for directing the assistant where to fix the 

 vane, by calling the division to him, especially when the reading 

 was near the top of the staff. A stud of wire, about half an 

 inch long, projects from the bottom of the staff, and a hole is 

 bored to receive it in the top of the peg which is driven into the 

 ground at every station, and on which the staff rests during the 

 observation. A small spirit-cup with a glass cover, screwed to 

 the lower part of the staff, serves to adjust it to a vertical posi- 

 tion, in which it is held fast by a clamp attached to three strong 

 legs, jointed and folding together, in the usual manner. 



The vane is a small mahogany box, about 3 inches in each 

 dimension, open at the. ends to admit the staff, which slides 

 through it. Two large wooden screws at the back of the vane 

 clamp it very firmly to the staff, and preclude all danger of 

 shifting. In front is a frame of brass, about 2 inches square, 

 sliding within an outer frame of brass screwed to the vane, with 

 a range of motion of about half an inch, either upwards or down- 

 wards, being moved by a large vertical screw with a milled head 

 working through the lower part of the outer frame. A square 

 aperture, corresponding with the inside of this frame, is cut 

 through the mahogany, in order that the divisions of the staff 

 may be seen. A small ivory door moving on a hinge, is fitted 

 into the sliding frame, on which are drawn two thick black lines, 

 crossing each other at a small angle, and a black ring with a 

 white circular spot within, at the centre, or intersection. At- 



