Sentence:  Although the tape was run for over 1 hr. , a steady state was not reached , and it was concluded that the reason for this was that the back pressure of the manometer was built up from the <WSD>material</WSD> fed from between the blocks and this was available at a very slow rate . 
Meaning of the word marked with <WSD> tags:  the tangible substance that goes into the makeup of a physical object