145 



•nd a manufac- 

 tory is eftablifh- 

 cd. 



The fpeftades 

 are called perif- 

 copic. 



GUN TO THROW DOUBLE-HEADED StfOf. 



Tlie moft advantageous proportions of curvature for obtain- 

 ing the different focal lengths now generally diftinguiihed by 

 certain numbers, have alfo been duly confidered; and the 

 nianufaflure of fpeflacles, on this conftru6tion, has been un- 

 dertaken by Meflrs. P. and J. Dollond, to whom the exclufive 

 fale of them is fecured by patent, and whofe well known ikill 

 in the conftru6lion of optical inftruments, enfures to this im- 

 provement every advantage of corre6l execution. 



From the opportunity afforded by thefe glafies of looking 

 round at various objects in one pofition of the head, it is 

 thought they may not improperly be diftinguiflied by the name 

 of Perifcopic Spectacles. 



XIV. 



Letter of Enquiry concerning a Gun to throw double-headed Shot; 

 ivith the Editor^s Reply i 



SIR, 



To Mr. NICHOLSON. 



de- Jnl, 



at the Royal 

 lililitutioii. 



Drawing andde- iJlAVING this inftant feen, at the Royal Inftitution, a (lieet 

 g'an'on'a paper of paper printed by Nicliols, printer. Earls Court, Newport 

 Street, Soho, containing a drawing and defcription of a gun to 

 throw double-headed fliot, I take the liberty of writing this on 

 a page of a little note-book there, which niuft be my apology for 

 the paper, to entreat that you will obferve upon it in your in- 

 valuable Journal, becaufe I fear that it is not a fafe inltru- 

 ment. A confiderable diftance is to be obferved between the 

 powder fired and the fliot to be difcharged by it. I have al- 

 ways confidered that an interval fo kept was the occalion of 

 the burfiing of fo many guns. Pray is this fafe? And fiiould 

 not a line be cut on the arms to which the bullets are attached, 

 to mark txadly the diftance of the attaching bar at each extre- 

 mity from the muzzle, when the balls are equally rammed 

 down. As the knowledge of its fafety may contribute to its 

 fervice if really ufeful, I hope you will think this application 

 not idle. 



I am. Sir, 



Your con flant R e a d e r . 



ANSWER 



