This is a letter written from Montreal, Canada in 1838, folded, addressed on the outside to Thomas Whitehead Senr, Rawtenstall, Nr Manchester, GB, and sealed with red wax.
This was normal at this time, envelopes being rarely used as they would increase the cost of postage, which was charged per sheet of paper, so an envelope would cost extra!
The simple address is also typical as far fewer people would send and receive letters. No stamp (not invented until 1840) and no street letter boxes, so it would have been handed to a clerk over the post office counter. There is a circular red Montreal postmark for April 10th at the bottom left.