2 9 2 SCIENTIFIC APPARATUS. 



used for the purpose a series of five to seven concentric circles, 

 in the middle of which was a depressed receptacle to contain the 

 needle (magrietinum indicem). The circles were divided, as is 

 the case to this day with many of the continental dials, into 

 twice twelve parts or " hours." For certain problems, sights were 

 taken upon strings stretched by plummet weights, and a graduated 

 quadrant with plumb-bob (libella stativa) was used for the purpose 

 of observing variations of level. 



About the middle of the last century the ordinary practice of 

 dialling or subterranean surveying appears to have been carried 

 on with but little change in the apparatus. " The instruments 

 used," says Dr. Pryce,* "are 'a compass without a gnomon or 

 style, but a centre pin projecting from the middle of the compass 

 to loop a line to, or stick a candle upon, fixed in a box exactly 

 true and level with its surface, about six, eight, or nine inches 

 square, nicely glazed with strong white glass, and a cover suitable 

 to it hung square and level with the upper part of the instrument ; 

 a twenty-four inch gauge or two-feet rule, and a string or small 

 cord with a plummet at the end of it ; a little stool to place the 

 dial horizontally, and pegs and pins of wood, a piece of chalk, 

 and pen, ink, and paper." A cord was held straight and tight 

 at the ends of the draft, the side of the compass was placed 

 " accurately parallel with the line," and the direction as given by 

 the needle end noted upon paper. 



The same author warns " those who take no account of the 

 points or angles of the compass, but in lieu thereof, chalk the 

 bearing of the line they measure with on the board the compass 

 lies in ; for if they are not exceedingly careful and precise in their 

 operations, they may commit almost unpardonable and irretriev- 

 able blunders : yet formerly, before penmanship and figures were 

 so generally understood and practised among the common tinners 

 as they are at present, most of our mines and adits were dialled 

 for in this manner." 



* " Mineralogia Cornubiensis," by \V. Pryce. London, 1778. 



