Creating A Running Total Calculated Field

I have a query, and I want a field that shows the current balance as of that transaction, eg:
__________________________________________________ _______
|__Transaction ID__|__Type_______|__Amount__|__Balance__|
|__1______________|__Deposit_____|__£10______|__£1 0_______|
|__2______________|__Deposit_____|__£30______|__£4 0_______|
|__3______________|__Withdrawal__|__£15______|__£2 5_______|

So far, I have this:

SELECT [Transaction ID], [Type], [Amount], (SELECT Sum([Amount]) FROM tblTransactions AS tblTransactions2 WHERE tblTransactions2.[Transaction ID] <= tblTransactions.[Transaction ID]) AS Balance
FROM tblTransactions
WHERE [Account ID]=1;


Although this does not look at the 'Type' field; it just adds the amounts; regardless of it being a deposit or withdrawal. I'm really not sure how to add this.

Thanks in advance.