280 SURVEY OF THE COAST 



cobweb on the bar with the imasi;e of the cross on the ivory, 

 is made by it, when tliis bar is to be placed for i;;oing off 

 again from tlie point determined by the position of the mi- 

 croscope. 



The piece O, bearing above the mother screw correspond- 

 ing to Q, has also a i)art X, going downwards through the 

 bottom of the box, which forms the socket of a steel axis of 

 two inches long and two-thirds of an inch mean diameter, 

 ascending vertically fiom the l)rass mother piece Y, which 

 runs upon the steel sciew ZZ, by which the box containing 

 all the ap|)aratus above described is moved horizontally in a 

 direction perpendicular to its length. 



The screw ZZ is held by a strong brass fraine which is 

 fastened to the end of a board of an inch and a half in thick- 

 ness, by screws exactly similar to those which fasten tlie 

 piece O to the box. At the other extremity of the box, a 

 strong circular socket is fastened by screws, receiving below 

 an axis Y, exactly equal to that described before, and move- 

 able in the same manner by another screw Z, equally fasten- 

 ed to the thick lower board Z'. 



This lateral motion is again eased by twenty-four brass 

 rollers GO, an inch in diameter and one incli and a quarter 

 long, fastened below the l)ottom of the box, and so tixed 

 that two are opposite to each other near the side of the box. 

 This motion being across the grain of the wood, there arc 

 screwed to the lower l)oar(l Z', tliin crossing pieces bb. of 

 very hard wood, and in the direction of the grain, upon which 

 rollers move with great ease, and without making any im- 

 pression. 



The box of this apparatus is always carried upon the thick 

 board Z', and jjlaced upon theliipods in a direction so near- 

 ly true, as to be within the limits of the screws ZZ : it is 

 then placed by these screws accurately in the true direc- 

 tion, being guided by a telescopic arrangement at the top of 

 the box, to be described immediately. 



The strong board Z', which supports the box with the bar 

 apparatus, rests upon five tripods with elevating screws in 



