Bell tents are manufactured from 100% cotton canvas and this is inherently waterproof. When the tent is used on the first few occasion the cotton fibres expand and knit together to create the waterproof fabric. If you are unfortunate enough to be subject to a torrential downpour on the first use of your tent you may experience small leaks particularly around the seam areas. This is perfectly normal and will subside after a couple of uses.
Generally the fibres expand in normal damp conditions and no leaking occurs. If looked after properly your Life Under Canvas Bell Tent should last years and years.