( 64 } 
Th^t SjJ^adrupeds Cannot ftand in their natural prone 
pofition on one or two feet, becaufe the Center of Gravi- 
ty and its line of propenfion cannot fall in either ^ or be- 
tween both. 
The reafon why in afcending or defcending Stairs in the 
dark, when weexped one more then there is, the exten- 
ded foot is daflied with violence againftthe Pavement, vi!(. 
In the firft cafe the Center of Gravity and Line of Propen- 
fion 5 are carryed forward beyond the foot that's fixt , Co 
thatthefufpendedfoor^ not meeting with the fupport ex- 
|).<5ted, a motion downward is neceflarily caufed by the 
gravity of the body :» according to that line^ which is not 
a ftep;» but a fall. I n the fecond cafe^the Mufcles of one leg 
are contradledj and lengthned in the othen but both thefe 
motions being checked by the unlookt-for refiftance of 
the Pavement, the like fenfation is caufed with what we 
feel when we ftamp on the grou nd. 
He (hews the Art of Seating upon Ice » as alfo how 
progreflion in Quadrupeds is performed > and likewife 
Leaping^ in which thez^/> motiva is to the weight of the ^ 
body as 2900 is to I . 
That in Leaping according to a line inclined to the Ho- 
n^ofz^ at oblique angles, the line defer ibed by the cen- 
ter of gravity fhall be a t'-z^rt'^ Parabola. 2LS being compoun- 
ded of the ftreight uniform motion forwards and the acce- 
lerated defcent of the Heavy body. 
Next he gives an exaft account of flying, the main 
ftrefs of which is in the largenefs of the mufcles that move 
the wings, the Potentia of which exceeds the weight ot 
the bird locoo times 5 with many more curious partl^- 
culars about their feveralwayes of flying. 
Hence he concludes the impoflibilityof the ^r^^^r^'^/ 
flying in Men^ for neither can their pe£toral mufcl^^ Be en- 
larged, nor the weight ot their bodysbedimiriilhed , tho 
attempted by an exceeding thin and large exhaufted recei- 
ver of Metal] 5 for it muft contain a fpaceequall to its own 
weight and that of a man faftned to it, which would a» 
mount 
