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 material fed from between the <WSD>blocks</WSD> and this was available at a very slow rate . 
Meaning of the word marked with <WSD> tags:  a solid piece of something (usually having flat rectangular sides)