keeping broody eggs moist

I have been reflecting on what happened with my latest broody clutch.   I think that the egg membranes became very dry and the chicks could not break through it.  Is there any way to prevent this next time?  In incubators I understand that people need to keep humidity levels up.  I was wondering about giving the broody eggs a mist spray daily  a few days before hatching.  Does anyone do this? 

I'm sure I read somewhere that if your using a broody, put a sod of earth under her nest and this can help with humidity, I could be wrong 

Just came back on to this thread and I really have to laugh  (Me autocensored) I will have to mind my language from now on 
What I meant to say was 'a square piece of turf cut from the earth' (I'm sure there must be a shorter word for this, Oh yes 'so and so')