New Machine far ruling engraved Plates. 431 



tl»e (Iroke mull firil be made. This is done by the lever O. If the fteel blade be dropped 

 Into the firft notch beginning on the left hand, the handle N will be confined ; if at the fc- 

 cond notch, the handle, upon being moved backwards and forwards between the pin T and 

 the lever O, will move the fcrew through one tooth, or one- fiftieth of a turn, each time, 

 and confequently will carry the ruler F througli one 2000th part of an inch. If the blade 

 of the lever O be placed in any other of the notches (which are numbered by filing the 

 corners of every fifth notch), the quantity pafled over, at each return of the handle, will be 

 greater or lefs according to the number. As there are but twenty- fix notches, the greateft 

 fingle fliift of this inftrument will be one-eightieth part of an inch ; but as the fliift is fo 

 readily made, it is eafy, even with this fine fcrew, to reach greater intervals, by moving the 

 handle once, twice, or even three times, between ftroke and firoke. Thus for one-fiftieth 

 of an inch, or ^ J-°^'^''**> ^^^ number of intervals cannot be pafled over at one fl;roke ; but, if 

 the blade be fet at the twentieth notch, the ruler will be fliifted exaftly that quantity by 

 two movements of the handle. It is needlefs to multiply diredtions for this part of the 

 operation ; and the more fo, as the artift is much more likely to determine the finencfs of 

 his fliadc by previous trials, which will render him acquainted with the inftrument, than by 

 4ny numerical computation. 



An inftrument like the prefent may alfo be ufefully employed in fubdividlng lines into 

 fmall parts. If curved lines be required to be drawn at equal diftances from each other, 

 the fimpleft method appears to be that of varying the figure of a temporary edge of horn 

 or metal, which may be fixed to E F : and if lines be required converging to a point, the 

 ruler E F may be made to occupy the radius of a circle of any magnitude whatever, by 

 giving an adjuftable angular figure to the Aider, and cauGng it to move againft two points, 

 upon principles deduced from the thirty-firft propofition of F.uclid's third book. 



As this inftrument was made at intervals of Icifure, without any account being kept of 

 time or materials, I cannot diftinflly ftate the coft : but I eftimate that the whole being 

 made of brafs and fteel, and well finiftied, with a fmall drawing-board and the wooden 

 holding piece I K, might be afforded by a ftiopkeeper in the mathematical-inftrument buG- 

 nefs for ten guineas. If the frame and Aider were of box- wood, which I have no doubt 

 would anfwer very well, and the ornamental covering LM of the ratchet-wheel were omit- 

 ted, I fuppofe the inftrument could be afforded for confiderably lefs than half that fum. 



The engraver has ufed this apparatus in the plates of the prefent number. 



II. 



Memoir on the Qimate of Ireland. By the Rev. JVjLLIAM HAMILTON, of Favet, in the 

 County of Donegal; late Fellonu of Trinity College, Dublin; M.R.I.A. Correfpondifi^ 

 Member of the Royal Society of Edinburgh, is'c 



[Concluded from Page 386, Vol. II.] 

 Of the EffeBs ofthefe We/fern Tempejls on the General Temperature of the Climate. 



I 



N a paper, honoured with a place in the fecond volume of the Tranfa£lions of the 

 Academy, I recorded the refult of fome experiments made in the year 1788, for the pur- 

 pofe of afceitaining the temperature of the earth in our latitudes. 



31^2 From 



