396 ' Defeription of a Ventilator 
Fig. 2, whereon to fecure the tube Fig. 4, when applied ‘to 
the nofle. The joints of the pipes, when put together for 
ufe, fhould be made air-tight, by means of bees-wax or fome 
ftronger cement, till they reach the bottom of the veffel, when 
there is no farther need of this precaution. | The horizontal 
pipes fhould run by the fide: of the kelfon the whole length 
of the hold. The tin plates of which K is made, fhould be 
punched in holes, like the rofe of a watering-pot, in two or 
three lines only at moft, and then formed into a tube, with 
the rough fide outwards. L may have four or five lines of 
the like perforations. M, and the reft, fhould gradually in- 
creafe in their number as they advance towards the middle of 
the hold, and continue fully perforated to the laft pipe, which 
fhould be clofed at its end to prevent the ingrefs of the corn. 
Tt is the centre of the cargo which moft requires ventilating, 
yet air fhould pervade the whole. Like the , trade-winds, it 
will direét its courfe to the part moft heated, and, having 
effected its falutary purpofe there, will difperfe itfelf to refreth 
the mafs, 
Where the hatches are clofe-caulked, to prevent the influx 
of water, vent-holes may be bored in convenient parts of the 
deck, to be bunged up, and opened occafionally, from whence 
the fiate of the corn may be known by the effluvia which 
afcend when the ventilator is working. 
The power of the ventilator is determined by the fquare of 
its diameter multiplied into the length of the ftroke, and that 
again by the number of ftrokes.in any given time. 
To find the area of a circle, and the folidity of a cylinder 
raifed on that circle, Archimedes gives the following propor- 
tion :— 
‘As 1 is to .785398 decimal parts, fo is the fquare of the 
diameter to the area of the circle. 
And, as 1 is to .785398, fo is the fquare of the diameter, 
multiplied by the height, to the folidity of the cylinder. 
The cubical contents, ‘both of eylinders and tubes, are 
found in the fame manner; their difference confifting not 
in fhape, but folidity, the latter being hollow. 
Then, to find the contents of a cylindrical veffel whofe in- 
ternal diameter is ten inches, multiply that into itfelf, and 
the 
