stock.txt is just the CSV file from Willis's XLS with heading text and lines with just ,,, removed. I've put it here

Here is my R program

v=as.matrix(read.csv("stock.txt")) a=matrix(NA,227,12) # First make a mat with 1 row per year a[v[,1]-1773,]=v[,2:13] # rub out first part with missing years b=a[-(1:27),] b=c(t(b)) x=1:length(b) e=lm(b~x) # dtrend b=b-e$fitted #b=a[-(1:27),] b=c(b,rep(0,2^16-length(b))) # pad x=1:length(b) u=abs(fft(b)) # fft x=1/x x=x/min(x)/12 # smallest period 1/12 year o=x<70 & x>1 # match x-axis and cut VHF png("tides.png",width=800) plot(x[o],u[o],ylim=c(0,10000),type="l",xlab="Period years",ylab="height measure") dev.off()