146 PHILOSOPHICAL TRANSACTIONS. [aNNO 1792. 



ing is to harden the metal, by which it becoiiies in some degree a non-conductor 

 of magnetism, and retains that rarefied and condensed, and therefore more sen- 

 sible, state of the fluid, which is produced by the influence of the earth's mag- 

 netic atmosphere. 



Exper. 18. In a small crucible Mr. B. placed 6 thin plates of copper, and be- 

 tween each of them a plate of zinc; these being melted, and cast in a proper 

 mould, produced an oblong piece of brass, which was not sensibly magnetic, nor 

 could he produce any magnetism in it by hammering. The same quantity of 

 copper and zinc were melted, with the addition of some small bits of iron. This 

 brass v;as very sensibly magnetic, and when hammered acquired polarity, by 

 which it more sensibly attracted or repelled the needle. Lastly, a piece of copper 

 was melted, with the addition of some iron, which was also sensibly magnetic. 

 From these experiments he concluded, that brass owes its magnetism to iron; 

 but that it may sometimes contain so small a quantity as not to be sensible till it 

 is hammered. 



FI. Purl of a Letter from Mr. M. Toppino, to Mr. T. Cavallo, F.R.S. p. 99. 

 DEAR SIR, Madras, Feb. 4^, 1 789. 



I inclose you an account of a base line I have measured for a series of triangles 

 I am carrying down the coast of Coromandel. I have already extended them to 

 about 300 miles from Madras, and am on returning back, to prosecute the work 

 quite down to Cape Comorin. 'The angles are all taken with my Hadley's sex- 

 tant, made by StanclifFe, by means of 3 tall signals I have constructed of bamboos, 

 80 feet high, 60 of which I mount on steps, so as to see (over all trees, &c.) 

 very distinctly my 1 other signals, at the distance of from 8 to 13 miles. It is, 

 I believe, the first time the Hadley was ever made use of for a purpose of such 

 magnitude; but it is fully equal to it — nay, it does more — the sun's bearing, or 

 oblique distance, from my signals is also taken by it; by which, and his azimuth 

 (computed) I obtain the angles made by them with the meridian; and by com- 

 bining the whole, the difference of latitude, and meridional distance of every 

 one of them in English fathoms. This is found so nicely, that a mean of my 

 astronomical observations for the latitudes, never differs more than a few seconds 

 from those given by the geometrical mensuration. In all the operations I have 

 had no one to assist me, except a party of black, fellows to carry my flags. I 

 need not tell you how many thousand miles I have travelled to take the angles; 

 nor what the labour and fatigue of such a work must be in this burning cli- 

 mate, where I have frequently had the thermometer at 106° in my tent. 



(Signed) M. Topping. 



On the Measurement of a Base Line on the Sea Beach, near Porto Novo, on the 



Coast of Coromandel, in May, J 788. By Mr. Michael Topping, p. 100. 



As a necessary foundation for the chain of triangles now carrying on, says Mr. 



