[R] First Derivative of Data Matrix

[thaumaturgy]

I am really new to R and ran across a need to take a data matrix and
calculate an approximation of the first derivative of the data.  I am more
than happy to do an "Excel" kind of calculation (deltaY/deltaX) for each
pair of rows down the matrix, but I don't know how to get R to do that kind
of calculation.  I'd like to store it as a 3rd column in the matrix as well.

My data looks like this:
     acflong
1  1.0000000
2  0.9875858
3  0.9871751
4  0.9867585
5  0.9863358
6  0.9859070
7  0.9854721
8  0.9850316
9  0.9817161
10 0.9812650

and I'd like to generate a table like this:

     acflong          dacflong/dx
1  1.0000000
2  0.9875858        -0.01241          #delta(acflong)/delta(index)
3  0.9871751        -0.00041
4  0.9867585        -0.00042
5  0.9863358        -0.00042
6  0.9859070        -0.00043
7  0.9854721        -0.00043
8  0.9850316        -0.00044
9  0.9817161        -0.00033
10 0.9812650       -0.00045

Is there a way to do this in R and how do I eliminate the first line of the
data?

Thanks,
-Chris
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