296 Mr Robertson on the Mechanical Arts of Persia, 



at the extreme end of either lever, is sufficient to shift the 

 stop-cock, and the time which elapses from the first contact 

 with the lever until it has performed its office, is only ten 

 minutes ; so that the friction on the clock only extends over 

 twenty minutes during the twenty-four hours, while, in the 

 old method, fig. 1, the lever I is exerting a much greater fric- 

 tion during the whole continuance of daylight. 



The method which I have employed in constructing the 

 dials, although no cheaper than those hitherto in use, possesses 

 the advantages of greater distinctness, and remaining much 

 longer clean. Each dial is formed of two discs of plate-glass, 

 on the outmost sheet, but on its inner surface, are painted the 

 hours and minutes in black japan. A thiri coating of white 

 japan, very finely levigated, is then laid over all, which gives 

 the dial the appearance of Bisque porcelain when illuminated, 

 and appears during day as if it were formed of glazed porce- 

 lain. The inner disc is then placed in contact with the outer, 

 and both fixed in their proper position. 



This method saves the great expense of the building system, 

 although the sheets of glass are nearly as expensive ; but it 

 keeps the dial clean ; the figures in the old method being made 

 of cast brass or lead, and raised externally, become receptacles 

 for dust, which each successive shower spreads over the trans- 

 lucent surface, to the evident detriment of its usefulness. 



On the Mechanical Arts of Persia. By James Robertson, 

 Esq., Civil and Mining Engineer, Edinburgh, late in the 

 service of the Shah of Persia. Communicated by the Royal 

 Scottish Society of Arts. With a Plate.* 



Although, perhaps, there is little to be gained in a practi- 

 cal point of view from a description of the Persian arts, it may 

 still be interesting to contrast our own highly improved ma- 

 nufactures with those of less advanced countries. 



Carpentry, — The art of carpentry, as understood in this 



* The papers, of which this is an abstract, were read before the Royal 

 Scottish Society of Arts on 14th Dec. 1040, 8th March 1841, and 28th Feb- 

 uary 1842, 



