208 REPORT—1840. 
mon on the hills above Greenock, and in other places in Scotland ; and 
Mr. Thom was led to adopt it from having seen its effect in nature 
in purifying dark moss-water which filtered through it. 
In forming the filters at Greenock, in 1827, Mr. Thom proved the 
effect of this substance, by breaking down the rock to the size of small 
peas and less, and mixing this with fine sharp sand, and then forming the 
filtering medium of the mixture. The fine sand is composed of quartz, 
reduced to powder by the action of the sea, &c. This sand keeps the 
filtering medium longer open than it would be without it, and is better 
adapted for the self-cleaning process than any other kind of sand, or 
than the pounded rock by itself. 
The water is filtered by passing directly downwards through the 
media; the media are in their turn cleaned by passing the water 
through them upwards. On the large scale it is proper to do this 
once a week ; but the whole process of cleaning only requires about an 
hour’s time, and to accomplish this it is only necessary to shut and 
open two stop-cocks alternately, and allow the water time to carry up 
any impurities that may be obstructing the operation; and these 
being always found very near the surface, the operation is quickened 
by moving the sand there gently with a fine rake, after the upward 
current has continued for a few minutes. If the surface is disturbed 
while the current is passing downwards, it does injury by sending the 
impurities further down. In Greenock the filtering bed is made con- 
siderably deeper than at Paisley, the water at the latter place being 
naturally less impure than at the former. The Paisley filter is, there- 
fore, much cheaper in the construction than those at Greenock; and 
this one filter, at an expense considerably under 1000/., produces an 
abundant supply of fine water for 30,000 inhabitants. 
On a Smoke Protector. By Mr. WAuvAce. 
On an Improved Working Barrel for use in sinking Pits, Se. 
By Marrutas Dunn. 
The improvement here referred to consists in attaching a branch side 
pipe of about one-third the diameter of the working barrel, extending 
from above to below the space traversed by the bucket, in which pipe 
is inserted a cock, whereby to regulate the discharge of water from the 
column above into the space below. It is peculiarly fitted to the exi- 
gencies of sinking pits, where the water is required to be kept so low 
in the bottom as to enable the sumping to be carried forward, and in | 
the effecting of which air is constantly liable to be drawn into the 
windbore at the snoreholes, which has a tendency to destroy the equi- 
librium of the engine and cause great and continual damage. The 
cock is manceuvred by the sinkers in the bottom of the pit by means 
of an iron rod, letting back so much water into the space between the 
bucket and clack as just to drain the feeders and nothing more, and 
by having a constant eye to it this is found to be exactly effected. 
