304 PHILOSOPHICAL TRANSACTIONS, [aNNO J 728. 



a wave-like motion to the medium immediately next to the surface gg; as that 

 vibratory motion is performed, the medium alternately pushes from the surface, 

 and returns towards it, as is represented by the position of the darts in the 

 figure, and pushes back the light, so as to reflect it when the vibration is con- 

 trary to its direction, but brings it down to be refracted when the vibration 

 conspires with the said motion. See a further account of this in Sir Isaac 

 Newton's Optics, Book 2, Parts, Prop. 12. 



The persons present at the experiments abovementioned, tried them as well 

 as myself, and being satisfied with the success of them, allowed me to mention 

 it, and make use of their names in this account. 



The Method of making Tin-Plates, extracted from the Memoirs of the Aca- 

 demy of Sciences, for the Year 1725 ; by JVilliam Rutty, M. D. R. S. Seer. 

 N° 406, p. 630. 



The making of tin-plates, or latten, as it is called, being not commonly 

 practised in England, though there is so great a consumption of it; either be- 

 cause the method is not sufliciently known, or because that in use to make 

 small quantities, for particular purposes, is much too dear to answer the arti- 

 ficer's expectation in making larger; by which we are obliged to export our 

 own tin to Germany, to receive it back again manufactured: it is thought not 

 improper to describe the method the Germans make use of, as extracted from 

 a Dissertation of Mr. De Reaumur, printed in the last volume of the Memoirs 

 of the Academy of Sciences of Paris, in which also he lays down some im- 

 provements, as he thinks, of his own. 



He takes notice then, that the making of tin-plates, called in France white 

 iron, properly begins with preparing the leaves or plates of iron to be tinned, 

 which are supposed to be sufficiently thin and flat, and cut into squares : but 

 it is only certain sorts of iron that can be reduced into these leaves, of which 

 those are the most proper, which when heated are easiest extendible, and yet 

 can be forged with a hammer when cold; the more soft and extremely flexible, 

 as well as the more brittle being rejected. These leaves are drawn from bars 

 of iron, about an inch square ; which being made a little flat, are cut into thin 

 pieces, then folded together, and being made into parcels containing 40 leaves 

 each, they are beaten all at once with a hammer of 600 to 700lb. weight. 

 After this, the principal part of the whole art is to prepare these leaves; for 

 the lightest dust, or the least rust on their surface, will prevent the tin from 

 uniting with them. This is taken off by steeping the plates in acid waters. 



