J. G. Galle's Measurements of the Rainbcm. 279 



the arrangement of the molecules of lead in the reduction 

 of the acetate, I employed, as is done to produce the tree of 

 Saturn, a slip of zinc placed in the centre of a broad spiral of 

 thick pure copper wire; the two upper emerging extremities 

 were soldered to form a voltaic couple. At the end of six 

 months or a year, the immersed portion of the copper wire, 

 originally very ductile, became so brittle, that it broke with 

 the least effort; the external portion nearly preserved its duc- 

 tility, at least it retained it longer. The brittle wire presented 

 a dull granular fracture, which indicated not only a different 

 molecular arrangement, but also that it had formed an alloy 

 by cementation with one of the elements of the solution. 



This same fact of the fragility of the wires subsequently oc- 

 curred again, on the occasion of my establishing a fixed elec- 

 trical apparatus above my house. These apparatus are formed, 

 as is known, of long copper wires stretched horizontally, in- 

 tended to draw away the electricity of the atmosphere to con- 

 duct it to the common centre, after having traversed a rheo- 

 meter. These wires, thus exposed to the changes of the sea- 

 sons, of atmospherical agents, and to the electric currents, 

 became brittle, and I was obliged to renew them at the end of 

 two years. Wires silvered over did not last longer; brass 

 wires broke at the end of six months. Sheltered conductors 

 retain their ductility much longer, but at last become brittle 

 under the influence of a permanent current. 



I purpose, the first fine weather, and as soon as my health 

 will permit me to go over the tops of the buildings, to renew 

 all the wires of my apparatus, substituting for them pure 

 copper wire well-annealed and perfectly tinned. I hope that 

 the layer of tin will be less permeable to external agents than 

 copper and silver, and that the cementation which assists the 

 effect of the electricity will be greatly diminished. 



I may add, in conclusion, that it is useless to stretch the 

 wires strongly, for too great a tension prevents their retraction 

 when they have been dilated, and the curve of the wire in- 

 creases at each change of temperature, until there is an equi- 

 librium between the two antagonist forces, the mechanical ex- 

 tension on the one part and the contraction on the other. 



XLI. Measurements of the Rainbow. By J. G. Galle*. 



Vl/^ITH reference to the investigations of Airyf and of 

 Miller J, I here communicate some distances of the 

 rainbow from tlie sun deduced in the year 184-3. 



* From PoggeiulorfFs Annalen, vol. Ixiii. p. 342. 



f Transactions of the Cambridge Philosophical Society, vol. vi. p. 379. 



X Phil. Mag. S. 3. vol. xviii. p. 520. 



