32 
POPULAR SCIENCE REVIEW. 
1,012 feet above the sea-level at the town of Idar itself, but at 
Oberstein, where it falls into the Nahe, its height is only 905 
feet. In the valley between the two towns, scarce a couple 
of miles apart, most of the agate-mills are situated. In 1867 
there were 153 mills, working 7 24 stones ; and though the greater 
number of these are in Birkenfeld, some are situated in the 
adjoining Prussian territory. 
Each mill contains from three to five stones, set on a hori- 
zontal axle, one extremity of which, passing outside the 
workshop, communicates with a water-wheel, by which the 
millstones are set in motion. Most of the older wheels are 
undershot, but overshot wheels are erected in the modem mills. 
Each wheel measures from 10 to 18 feet in diameter. As the 
working is dependent on a due supply of water it formerly 
happened that the mills were compelled to stand idle during 
the drought of summer or the frosts of winter ; artificial pro- 
vision is, however, now made for a supply of water during the 
dry season, and the use of steam has been introduced to a 
limited extent. When the writer last visited Oberstein, a 
small engine of 1 6-horse power was working in the mill of the 
Grebruder Purper in Idar; this engine gave motion to five 
millstones, accommodating ten men, but was capable of turning 
eight stones, employing sixteen grinders. Another engine was 
working at Herrstein. 
The millstones are made of red sandstone from the Bunter of 
the neighbourhood of Landstuhl, near Mannheim. Each wheel 
is about five feet in diameter, and rotates in a vertical plane ; the 
lower half of the wheel moving in a well beneath the floor of the 
workshop (fig. 3, PI. II.). The broad edge of the wheel is kept 
moist by a stream of water constantly trickling down from a 
launder running above the series of stones. Before being brought 
into use, the wheels are seasoned by exposure for some time in 
the open air. Stones fresh from the quarry have been known 
to suddenly fly to pieces while rapidly rotating ; and on two 
occasions grinders have thus been killed in the Oberstein mills.* 
The stones generally make three revolutions per second. 
When choice stones are to be worked, it is usual to begin 
by slitting them into shape with steel wheels and diamond 
powder. The commoner agates, however, are not sawn, but 
roughly dressed with hammer and chisel, the workman 
acquiring by long practice great dexterity in striking the 
stones in the proper direction to insure the desired fracture. 
The grinding is effected on the broad edge of the red sandstone 
wheel, which is furrowed with channels corresponding in shape 
* Description of the Agate Mills at Oberstein in Germany. “ Mechanic’s- 
Magazine,” 1823, vol. i. p. 199. 
