SEXTANTS AND OTHER REFLECTING INSTRXJMENTS. 53 



of easily procuring copper in a state of purity ; and it is with the metal so pro- 

 cured my experiments have been made. 



By compounding copper and tin in their atomic proportions of 16 parts of 

 copper to 14.92 parts tin, a metal of high lustre is obtained ; and, so far as my 

 experiments have gone, this metal is not liable to tarnish, if ordinary care be taken 

 to guard against this effect. My course of procedure was as follows : — I first ex- 

 posed polished pieces of this metal to the free open air, and found, after some 

 months' exposure, that, when the dust and rain stains were rubbed off, the surface 

 was in a very good state of preservation. I next tried exposing the mirrors to 

 the fumes of acids, and watering them with sea water for a considerable length of 

 time. Under the operation of these corrosive agents, they still retained their 

 lustre. But, notwithstanding the encouragement held out from these experiments, 

 feeling yet reluctant to put such instruments into the hands of navigators without 

 first submitting them to the test of actual service ; and a favourable opportunity 

 having last spring presented itself, through the kindness of Mr 0. Mobsman, one 

 of the surveyors on board of H.M. Ketch, Sparrow, then about to engage in a sur- 

 vey of the Pentland Firth, I put into the hands of that gentleman a sextant 

 fitted with these metallic reflectors, of which he politely took charge, promising 

 to give it a fair trial during a season's survey. Mr Mossman amply redeemed his 

 promise ; and I shall now take the liberty of quoting the letters which he was good 

 enough to address to me, giving an account of the working of the instrument. 



" H.M. Ketch, Spaerow, 



THL'nso, 26th October 1844. 



" Dear Sir, — After you have examined the reflectors you will be able to 

 judge of the durability of them, after being in constant use for most part of the 

 season, during which, they have been exposed to all sorts of weather. As regards 

 the power of their reflection (although dark), they are beyond all the silvered 

 glasses I have ever had in use ; also, for measuring large or small angles, they 

 excel the others by far. When we are once properly settled at Portsmouth, I 

 shall be able to make a fuU descant of the good properties of your metallic re- 

 flectors, and shall strongly recommend them to be in all reflecting instruments. 

 I have subjected them to aU exposure which I could call fair play, only, at the 

 same time, having been careful not to put the instrument away damp. — Yours 

 truly, (Signed) " Wm. 0. Mossman." 



In a second letter, dated Portsmouth, 13th December, Mr Mobsman says : — 

 " I am sorry my spare moments are rather scarce, or I should have said some- 

 thing more concerning the merits of your metallic mirrors. However, I should 

 very strongly recommend them to be used in all instruments that are likely to be 

 exposed to much wet ; because, if the instrument faU overboard, for instance, and 



