Re the postmark and delivery of the letter; the environment around the camp; their mother and other expedition members have been badly bitten by mosquitoes and other insects and are worn down; unloading the ship; the weather and aurora; arrival of the observing equipment; bathing in the lake, etc.
Postscript: thanks for their letters; blessings; Frank has 'been simply of immense use to us, and has worked like a Briton'.