296 MARITIME OBSERVATIONS. 
menfions of the great fail, and expofing a quarter of its 
furface to the wind, would give a quarter of the force; fo 
that the whole force obtained from the wind would be the 
fame, while the refiftance from the air would be nearly 
reduced to the fpace between the pricked lines aband cd, 
before the foremoft fail. 
It may perhaps be doubted whether the refiftance from 
the air would be fo diminithed ; fince poflibly each of the 
following fmall fails having alfo air before it, which muft 
be removed, the refiftance on the whole would be the 
fame. ‘ 
This is then a matter to be determined by experiment. 
I will mention one that I many years fince made with 
fuccefs for another purpofe; and I will propofe another 
fmall one eafily made. If that too fucceeds, I fhould 
think it worth while to make a larger, though at fome 
expence, on a river boat; and perhaps time and the im- 
provements experience will afford, may make it applicable: 
with advantage to larger veffels. 
Having near my kitchen chimney a round hole of eight 
inches diameter, through which was a conftant fteady 
current of air, increafing or diminifhing only as the fire 
increafed or diminifhed, I contrived to place my jack fo 
as to receive that current; and taking off the flyers, I fix- 
ed in their ftead on the fame pivot a round tin plate of 
near the fame diameter with the hole; and having cut it 
in radial lines almoft to the centre, fo as to have fix equal 
vanes, I gave to each of them the obliquity of forty-five 
degrees. They moved round, without the weight, by the 
impreffion only of the current of air, but too flowly for 
the purpofe of roafting. I fufpeéted that the air {truck hy 
the back of each vane might poflibly by its refiftance re- 
tard the motion; and to try this, I cut each of them into 
two, and I placed the twelve, each having the fame obli- 
quity, ina line behind each other, when I perceived a great 
augmentation in its velocity, which encouraged me to di- 
vide 
