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of the second and fourth are alike, but reverse withrespect to the other two.

By connecting the first and second group with ametallic plate laid upon their uppermost metals, thetwo become one series, having its copper end at thebottom of the first group, and its zinc end at thebottom of the second one. In precisely the samemanner the third and fourth groups are united, andform another distinct series ; and by uniting thesecond and third groups at the bottom, the whole ofthe four groups become one uniformly continuedseries of compound elements, from the bottom of thefirst to the bottom of the fourth group.

By a similar distribution of the elements, a series ofany required extent might he formed so as to operatein concert, without spoiling their insulation bysqueezing out the water by superincumbent pressure ;and when the metals are soldered together in pairs,which is now the usual practice, no cleaning for con-tact is required. Gold and silver coins have longbeen discontinued in the structure of Voltaic piles ;copper and zinc are now the usual metals employed,and they are as frequently in square plates as inround ones.

The first modification which the pile received wasby an ingenious contrivance of Dr. Cruickshank, ofthe Royal Military Hospital, at Woolwich, in theyear 1800, a few months after the pile became knownFig. 22.

in this country. Fig. 22 will afford a good idea ofthe Cruickshank’s trough, or battery, as it is usually