SOME REFINEMENTS OF MECHANICAL SCIENCE. 147 



Of the several engines eonstrticted later the one most successful 

 and representing the greatest progress was that made by Rauisden 

 in 1777. This engine, automatic in its movements, was made espe- 

 cially for graduating circles, and because of the great precision with 

 which he divided the circles of the instnmients used by the Govern- 

 ment the board of longitude awarded him the sum of £615. A fur- 

 ther and most potent recognition of the excellence of his work lies in 

 the fact that all subsequent circular dividing engines have followed 

 closely the same general principles of construction embodied in the. 

 Ramsden engine. 



It is most gratifying to all those who are interested in mechanical 

 progress that the Ramsden engine has been preserved throughout all 

 these years and now stands in the Museum of the Smithsonian Insti- 

 tution at Washington as a monument to the one who made it and as 

 the best example of that time of the art of graduating circles. 



Many excellent dividing engines have been made that are quite 

 sufficient in point of accuracy for the work for which they were 

 intended, but the perfection required in the graduation of circles for 

 astronomical instruments is such that it has been found to be one of 

 the most difficult of all mechanical problems to make an engine that 

 will meet such requirements. 



In such an engine the chief essential is that the spindle carrying 

 the master plate shall be as nearly round and as closely fitted in its 

 bearings as is possible, for the degree of excellence with which that 

 work is done determines how closely a circle can be divided. 



It seems almost incredible that a well-lubricated spindle of 4 

 inches in diameter at its largest part and tapering three-quarters of 

 an inch to the foot can be made so nearly round and so closely fitted 

 in its bearings that a movement of one-thousandth of an inch in or 

 out of its bearings will in one case cause it to turn with difficulty 

 and in the other with perfect freedom ; yet this has been found to be 

 within the limits of mechanical refinements. 



The greatest accuracy thus far attained in such engines is one 

 second of arc, which arc, with a radius of 3 miles, equals 1 inch, and 

 at 20 inches, which is the radius of the silver ring upon which the 

 graduations on the master plate are made, a line one-thousandth of 

 an inch in width is equal to twelve seconds of arc, or twelve times the 

 accumulated errors of any number of divisions, or twenty times the 

 greatest error of any single division. 



In automatically graduating a circle, it has been found to be 

 impracticable to cut more than six lines in a minute, and it requires 

 about thirty-three hours to divide a circle into two-minute spaces. 

 As with the running of the finest clocks, so only can the best results 

 be obtained when the engine is surrounded with every favorable 



