the Rejiflance of the Air. 14* 
the table at the end of this paper, the meal ures and weight! 
are fet down exaCtly. 
Dr. hook, whofe name mud be refpeCted by every experi- 
mental philofopher, was aware, that although he thought he 
could demon (Irate that flat fails were preferable to fuch as were 
curved and hollowed by the wind, yet until proper experi- 
ments had been tried, nothing could be pofltively determined. 
He i ays lomewhere in his pofthumous works, 44 That he 
64 was lurprifed at the obdinacy of Teamen, in continuing, 
44 alter what appeared the cleared: demondration to the con- 
44 trary, to prefer bellying or bunting fails to fuch as were 
44 hauled taught; but that he would, at fome future time, 
44 add the tell of experiment to mathematical invedigation.” 
He reafoned upon a l'uppolition, that the air in. motion followed 
the fame laws as light ; and. that it was reflected from furfaces 
with the angle of reflection- equal to the angle of incidence, 
which is not the cafe, as it never makes an angle with the. 
plane, but is always reflected in curves. Monf. parent, and 
other mathematicians, have fallen into the fame miftake. No- 
demon ft ration of this fort was more commonly known or 
received among practical mechanics, than that the beft angle 
for the fails- of a wind-mill, at the beginning of their motion, 
was an angle of forty-five degrees ; and that the maximum of 
an under-fliot water-wheel was when it moved with one-third 
of the velocity of the water: but Mr. smeaton, in an excel- 
lent paper in the Philofophical TranfaCtions, has refuted this 
opinion by the cleared: experiments. 
I had intended to diverfify thefe experiments, and to extendL 
them to a more interefling fubjeCt of enquiry, to determine 
the bed: fhape of fails, and the angle to which they (hould be 
fet, to obtain the greateft progreflive effeCl with the lead lee- 
way ; 
3 
