first it was feared that a small quantity of mud would be deposited
on the surface of the old sand, and that this mud would ultimately cause
subsurface clogging. For this reason, when this method was first
adopted, a man was required to rake the sand very thoroughly in front of
the discharge. Later, it was found that by giving the end of the
discharge pipe a slope of about 45 degrees downward from the horizontal,
the force of the current of sand and water could be depended on to cut
the old surface of sand to any required depth, and move it ahead
together with the new sand, thus completely breaking up the possible mud
layer between the old and new sand layers. After having used this method
almost exclusively for 15 months, in which time eleven filters have been
re-sanded, and 24,531 cu. yd. of sand have been replaced, there seems to
be no indication of an increased initial loss of head. The sand is very
compact, and has no apparent tendency to separate into different sizes.
The general appearance is similar to that of very fine sand on the
seashore. The filters re-sanded in this way have been considerably more
efficient than those in which the sand was replaced with carts, and as
yet, no harmful results have been noted. The rate at which the sand is
replaced is shown in Table 12, and the cost of labor for sand handling
is given in detail in Table 14, which shows that quite a perceptible
saving has been effected by the hydraulic method.

The figures showing the cost for sand handling do not include any
charge for the quantity of water used, that item having been carried
on the pumping-station account.

~Table 11--Quantities of Sand Removed.~
==============+=================+================================
|~No. of filters | ~Cubic yards when last
|scraped when last| treatment was:~
|treatment was:~ |
Month. +---------+-------+---------------+----------------
| | | Scraping. | Raking.
|Scraping.|Raking.|----+----+-----+----+----+------
| | |Max.|Min.|Ave. |Max.|Min.|Ave.
--------------+---------+-------+----+----+-----+----+----+------
1909.
--------------+---------+-------+----+----+-----+----+----+------
July | 14 | 0 |338 |121 |190.6| 0 | 0 | 0
August | 8 | 0 |356 |149 |218.5| 0 | 0 | 0
Septe