We have measured 21-cm absorption spectra in the direction of three southern pulsars , PSRs B0736–40 , B1451–68 and B1641–45 , in three separate epochs spread over 2.5 yr. We see no evidence for any changes in the absorption spectra over this time span in spite of good velocity resolution and sensitivity .
Towards PSR B1641–45 we place an upper limit of 10 ^ { 19 } cm ^ { -2 } on the change in the column density of the cold , neutral gas .
In addition , we observed PSR B1557–50 and compared its H i absorption spectrum with spectra taken in 1980 and 1994 .
A prominent deep absorption feature seen at –110 km s ^ { -1 } in 1994 is weaker in 2000 and likely was not present at all in 1980 .
Using the standard interpretation which links the distance traversed by the pulsar to a physical cloud size , this results in a cloud size of \sim 1000 AU and density of 2 \times 10 ^ { 4 } cm ^ { -3 } , parameters typical of those seen in other observations .
These results are also consistent with an alternative model by Deshpande ( 2000 ) who expects the largest variations in optical depth to be seen against the longest time intervals .