112 LECTURE X. 



ploy; and those whq have continued the extensive survey which he began, 

 even prefer it to every other. For the comparison of standards, and for de- 

 termining small distances with great precision, beam compasses, or scales 

 with sliding indices, furnished with microscopes and cross wires, have been 

 constructed by the artists of this country : in France, a lever has sometimes 

 been introduced, its longer arm having an ample range of motion, corres- 

 ponding to a very minute difference in the length of the substance which 

 acts on the shorter arm. But for common purposes the diagonal scale is suf- 

 ficiently accurate, and may be applied without the error of the thousandth 

 of an inch: in cases where a very delicate vernier, or a micrometer screw is 

 applied, a magnifier is usually required. Mr. Coventry has, however, suc- 

 ceeded in making simple scales, which are accurate enough to measure the ten 

 thousandth of an inch. lie draws parallel lines on glass, at this distance, 

 which are in some parts sufficiently regular, although they can only be seen 

 by the help of a powerful microscope : but those which are at the distance of 

 the five thousandth of an inch are much more correct and distinct. For di- 

 viding rectilinear scales of all kinds, ]\Ir. Ramsden constructed a machine 

 which acts by the turns of a screw: others have employed an apparatus re- 

 sembling Marquois's parallel rulers. (Plate VII. Fig. 95 . . 97') 



The motion of a ship at sea is measured by a log line, or a rope divided by 

 knots into ecjual parts, and attached to a log, which is retained nearly at rest 

 by the resistance of the water. Attempts have also been made to cause a 

 little waterwheel to turn by the motion of the ship, and to measure both the 

 rate and the distance run ; and an instrument has been invented for doing 

 the same upon hydraulical principles; raising the water of a gage to different 

 heights, by means of the pressure occasioned by the relative motion of the 

 jship and the water, and discharging at the same time a small stream into a 

 reservoir, with a velocity proportional to that of the sliip. 



