Ferret command line just as internal Ferret functions (e.g. SIN, COS) are invoked. For example, you might create a routine to compute the amplitudes of the Fourier transform of a time series (the periodogram) and name your function "FFT_AMP". In Ferret you would use it like this:</p>
<pre>LET my_fft = FFT_AMP(my_time_series)</pre><p align="left">Once the variable my_fft is defined, it can be used in other expressions, plotted, etc. External functions can be used in every way that Ferret internal functions a