( -ifsi ) 
' come to the Wing : And when the Wings come near to 
the Apcrfare, they drive their new Air into the Aperture 
without any fridlion 5 and the Air which hath been firft 
driven and removed from the Wing, cannot lofe its fwift- 
^. nefs, becaufe the Wings which continually follow do con- 
tinually drive new Air, which keeps that which is before 
always in the fame fwiftnefs. This new (hape of the Hef- 
fiau Bdlows affords alfo another advantage ^ becaufe the 
Air in going round follows the Spiral line, which is 
nearer to the ftraight line than a Circular circumference 5. 
and when the Air comes to the Aperture, it gets into it 
without ^ny lofs of fubftance^ but in the Gylindrical Ma- 
chine, Fig. 2. the Air doth always go round in a Circular 
circumference 5 and when it comes to the Aperture, the 
Wind is driven direftly in the direftion of the Tangent 
but juft in the beginning at and afterwards the im^ 
pulfion is oblique : And this obliquity is always increafing 
until the Wing comes to the punSum A : Now it is known 
how much diminution fuch an obliquity can make to the 
ftrength. I believe therefore th^t this Spiral figure is a 
good improvement to this Engine. And indeed I have 
made fuch Bellows where the Radms- A¥ is but 10 i 
inches, the Wing Am 2 inches broad and 9 inches high^^ 
becaufe the Tympmum is alfofo high^ or little more 5 the 
Aperture AB is alfo 9 inches, or a little more, fo that it 
makes a fquare hole. When I work this Engine with mf. 
Foot, it makes fuch a Wind that it may raife up two 
pounds weight 3 and without doubt a ftronger Man could 
do much more-: But this is more than fafficient for our pur- 
pofe, fince we muft but drive Air enougli for the refpira- 
tion of fuch Men that can work in the Mine 3. and we 
may eafily with Boards make Wooden Pipes, to carry the 
Wind to the very bottom : So that the Air within will be. 
continually renewed as well as without. 
His Serene Highnefs being gone to Shudbdch, Fmufl 
©xpea his return to apply the Engine to uie Mine j and I: 
