 being located on top of a natural hill site whose slope directs {HEAD ~ the flow} in an upward direction, so that the said angular relationship between the helicoid axes and the flow vector is secured on {TAIL ~ account} of the chosen location 
being located on top of a natural hill site whose slope directs { HEAD ~ the flow } in an upward direction , so that the said angular relationship between the helicoid axes and the flow vector is secured on { TAIL ~ account } of the chosen location
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH