114 Description of an improved 



The sketch underneath will illustrate this subject. 

 . E represents the earth ; M the moon ; 00 part of the moon's 

 orbit. 



ab c the passage of the projectile in a parabolic curve toward 

 the earth. 



Its greatest altitude is marked in its covirse. Its extent of arc iu 

 a parabolic curve of very moderate eccentricity ; the proportional 

 size of the earth and moon to each other, and to their distance 

 nearly. The whole I think will prove how utterly void of appa- 

 rent foundation the idea is of these projectiles from the moon 

 reaching us. The semicircle round the earth includes a radius 

 of about (JOOO miles. 



M M radius at which the projectile might revolve round the 

 moon. 



XIX. Description o/"JV/r. Edmund Turrell's improved Draiv- 

 ing Board and T Square-'^. 



Sir, — J. HE invention of ruling machines in the art of copper- 

 plate engraving, produced such a degree of perfection in the tints 

 ruled by them, that a corresponding degree of accuracy was 

 immediately required in all the other departments of the art, 

 wherever the use of the machine was introduced. 



But unfortunately for the credit of this department of the art, 

 a most general o])inion was directly formed, that engraving done 

 with a machine required but little exertion in the artist to attain 

 perfection, and that nearly the whole secret lay in the possession 

 of a ruling machine : — Experience has most indubitably proved, 

 that in general practice, the invention of a ruling machine, at 

 the same time that it produced a portion of perfection in the 

 tints ruled by it, never before seen, presented a degree of diffi- 

 culty in the drawing and finishing department, such as had never 

 been experienced. Indeed, such an excess of difficulty as might 

 have prevented its adoj)tion in the hands of any other person 

 than that of its most ingenious inventor. 



One of the most formidable difficulties which presents itself to 

 an engraver of machinery and scientific subjects, is that of getting 

 a correct outline tracing upon the copper-plate. To effect this, 

 transparent oiled paper (commonly called tracing paper) is used, 

 for subjects of the free picturesque kind. But in plans, elevations, 

 and sections of buildings, machinery, &c. such means are of lit- 



* From the Transactions of the Society for the Encouragement of Arts, 

 Manufactures, attd Commerce, vol. xxxiv. — For these improvements Mr. 

 Turrell was voted the Society's silver medal, and five guineas. 



tie 



