[R] Subtraction loop

[ecvetano at uwaterloo.ca]

I have a data frame with 2 columns, one for day and one for average.  
The day starts at 97 all the way to 279. I want to subtract day 98  
average- day 97 average, then day99 average -day 98 average and so on  
down my list, creating another column with the subtracted results.
I have:

     Day DailyAverage
1    97    0.6076782
2    98    0.7121478
3    99    0.8059347
4   100    0.9545806
5   101    1.0589791
6   102    1.1335981
7   103    1.2676922
8   104    1.4429847
9   105    1.6477266
10  106    1.7970784
etc. with values to day 279

I'm guessing I need to create some kind of loop for this, but I am  
unsure as to how to do this. Any help is greatly appreciated.

Thanks,
Emilija