On the Food of Blints. 317 
Mr. Woolf’s improvement on the piston is such as to 
enable the engine-man to tighten the piston without the 
necessity of taking off the cover of the cylinder, except when 
new packing becomes necessary. He accomplishes this by 
either of the two following methods : 
He fastens each of the screws into a small wheel (c, fig. 1, 
and c,c,c,c,c, fig. 2. Plate VI.), which are all connected 
with-each other by means of a central wheel (d,d,}, which 
works loose upon the piston-rod in such a manner, that if 
eone of the small wheels be turned, it turns the central wheel, 
and the latter turns the other four. The one that is to be 
first turned is furnished with a projecting square head, which 
rises up into a recess in the cover of the cylinder. This re- 
cess is surmounted by a cap or bonnet e (Plate VI. and VIT.), 
which being easily taken off, and as easily put again in its 
place, there is little difficulty in screwing down the packing 
at any time. ‘The parts are so clearly expressed in'the plates 
that no further description is necessary to make any person 
comprehend it. 
The other method is similar in principle, but a little dif- 
ferent in construction. (See Plate VII.) Instead of having 
several screws all worked down by one motion, there is in 
this but one screw, and that one is a part of the piston-rod ; 
on this is placed a wheel of a convenient diameter, the centre 
of which is furnished with a female screw. This wheel is 
turned round, 7. e. screwed down by means of the pinion 0,0, 
which is furnished with a square projecting head rising into 
a recess of the kind already described. The ring is prevented 
from turning with the wheel by means of two steady pins. 
L. On the Food of Plants.. By the Rev. Josrra Towns- 
HEND, Rector of Pewsey, Wilts*. 
W war is the food of plants ?—Before we can give a satis- 
_ factory answer to this question we must collect facts, we 
must multiply experiments. Tor this purpose, in the years 
* From Letters and Papers of the Bath Agricuttural Sucisty, vol. x, ., 
1792 
