QlTEIllES ON THE ARTS, J30 



II. 



Enquires respecting various subjects relating to the Arts. Bu./ 

 JyvENis. With some remarks by W. N. 



To Mr. NICHOLSON. 

 SIR, 



jL our kind attention to your correspondent R. B. vol. vii. 



71, has induced me to trouble you with the following queries, 



not doubting but that from you or your learned corresppn- 



dents, I shall receive a satisfactory answer to them. 



1st. Is there any rnethod of freeing iron boilers from the in- Query 1.— Rev 



crustation left on them by hard water, without subjecting them -^^i^yg^f^j^^ 



to a red heat. Copper vessels, I know, are readily so cleaned, left by 'vater 



but cast iron will not stand that heat without cracking. "P°" ''"''" '^'*^'- 



2d. Is it possible to reduce printing ink to a fluid state, so Query 2.— On 

 ,„ . . , „ , , . , . ., the taking off 



as to take a copy of a copper-plate, or prmred paper, similar ^Qy^^gip^^^fg 



to the method used in copying writing, and this without the from engrav- 

 original receiving any other damage than that of becoming ° ' 

 fainter, which the loss of the ink must inevitably occasion. 



od. Is there any method of ascertaining the degree of poro- Query3. — Im- 

 sity of the metals of reflecting telescopes, except by their pei"- g^eculums* *^ 

 formance when polished ? Edwards says the porosity arises 

 from the cajcination of the tin, and I have tried a number of 

 \vays to prevent this, but am at a loss which to prefer, for want 

 of some criterion whereby to judge of the degree of porosity 

 in each separate metal. When the tin is left lung in the fire, 

 the metal I know will be bad, but I cannot (when well polish- 

 ed), even with the highest magnifiers, discover any pores in it, 

 though I have no doubt but the badness of its performance 

 arises from that cause. But the performance alone cannot 

 certainly indicate the degree of porosity, since I find metals of 

 the same composition, when ground and polished in every re- 

 spect the same, seldom or never exactly agree in performance. 



4th. In a late edition of Ferguson's lectures by Brewster, Query 4.— Sup^ 

 the following rule is given for making the eye-pieces of telescopes P°^<;^ achro- 

 achromatic. With two lenses, the focal length of the first three glass. 



times 



