86 LECTURE X. 



A variety of instruments are used for the immediate comparison of the 

 standard measure or its parts, with other lengths or distances. Such are 

 scales, simple and diagonal,* verniers, micrometer screws, beam compasses, 

 rods, lines, chains, and measuring wheels. The greatest accuracy has 

 generally been supposed to be obtained, in large distances, by means of 

 rods, made of glass or of platina, in order to be less susceptible of such 

 changes as are produced by variations of temperature ; General Roy,t how- 

 ever, found that a steel chain was as little liable to error, as any mode 

 that he could employ ; and those who have continued the extensive survey 

 which he began, even prefer it to every other. J For the comparison of 

 standards, and for determining 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, corresponding 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 sufficiently 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, succeeded in making simple scales 

 which are accurate enough to measure the ten thousandth of an inch. 

 He 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 power- 

 ful microscope : but those which are at the distance of the five thousandth 

 of an inch are much more correct and distinct. For dividing rectilinear 

 scales of all kinds, Mr. Ramsden constructed a machine which acts by 

 the turns of a screw : others have employed an apparatus resembling Mar- 

 quois'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 equal 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 ship and the water, and discharging at the same time a 

 small stream into a reservoir, with a velocity proportional to that of the 

 ship. 



LECT. X. ADDITIONAL AUTHORITIES. 



Drawing and Painting. Leonardo da Vinci, Trattato della Pittura, 4to, Rome, 

 1817; Translation by Rigaud. Junii de Pictura Veterum, fol. Rotterd. 1694. Du- 

 fresnoy, Art of Painting. De Piles, Do. 1706. Bardwell, Do. 4to, Lond. 1706. 



* Hooke on Diagonal Divisions, Animad. on Hevelius. Wallis on Do. Phil. 

 Tr. 1674, ix. 243. 



f Roy's Account of the Measurement of a Base on Hounslow Heath, Ph. Tr. 

 1785, Ixxv. 385. 



J Ramsden's Steel Chain, Ph.Tr. 1785, p. 394. 



Ramsden's Description of an Inst. for dividing Lines, 4to, 1779. 



