[R] How to choose a button and scrape the website data

[Guang Dai]

hi all, 
I'm working on scrapping some website data to build a database. 
Under most cases, I can use package XML to get the dataset. 
However, some of the website doesn't give a explicit address of the downloaded tables. 

To be more specific, for example, I'm interested in the website http://ets.aeso.ca/
The data we are scraping is the "Pool Weekly Summary" under the category of "Historical". 
However, after clicking "historical" and choose the "Pool Weekly Summary"  item on the website,
the address is always http://ets.aeso.ca/ and doesn't change. 

In this case, I guess I need to tell R first click the "historical" button then choose the item before 
scraping the data. But, the question is how?

Any suggestions are welcome. 
Guang