TBitmap.Canvas.TextWidth problem

assuming i created a TBitmap (bm) as global variable on top ....then,

function TForm1.test();
function getTextWidth(var bm:TBitmap):integer;
var s:string;
var b : tfont;
begin
  s := 'From TransNo: []   To TransNo: []     From Salesman: []   To Salesman: []     From Date: [01/10/2007]   To Date: [31/10/2007]';
  b := TFont.create;
  b.Name := 'Times New Roman';
  b.Size := 8;
  bm.canvas.Font.Assign(b);
  result  := bm.canvas.textwidth(s);
end;

begin
  aWidth := getTextWidth(bm);
  .....
  ..... (other source code)
  .....
  .....
  aWidth := getTextWidth(bm);
end;

at first time, my aWidth value = 595, while the 2nd time , it return 1950; the source code in between have changed my TBtimap property ...is there anyway i can list out all properties of TBitmap object (e.g. size, width ...) so that i can compare what have been changed before and after ...?

thanks