Sentence:  6thly, That there shall be prayers twice a day at due times to be appointed by the schoolmaster (holidays and sickness, or any other needful and <WSD>necessitous</WSD> cause to be shewn to the contrary) and to be said by him in the school house adjoining the alms-houses; at which prayers, I ordain, that all the almswomen of the said almshouse be duly and daily there present (except they be hindered by reason of sickness, or some other tolerable cause to be admitted) of whom the schoolmaster was to examine into the same, wherein I desire him to be very careful to observe. 
Meaning of the word marked with <WSD> tags:  Necessary; unavoidable.