( i6o ) 
the Years 1666 and 1667, but which he himfelf did 
not feem much to rely on. Now, both thefe Periods 
may be very conliftent with the fame Obfervations, 
provided that one of the Obfervers did not continue 
his Obfervations for any confiderable Time at once. 
For if the exad Situation of any Spot be obferved 
at any given Hour one Day, and at the fame Hour 
the fuGceeding Day be found advanced about ly De- 
grees or sV of the whole Revolution, it may ftill remain 
doubtful, whether the Spot has moved only thro’ thofe 
ly Deg. in that Day, or has made one or more en- 
tire Revolutions belides in that Time. This the Au- 
thor was aware of, and therefore waited for an Oppor- 
tunity of attending to the Motion of a Spot as long 
at once as the Vicinity of Venus to the Sun would ad- 
mit of. Accordingly, Feh. 1716, a little after Sun- 
fet, he obferved a Spot near the Center of her Disk, 
where their Motion is moft perceptible in a (hort Time, 
and about 3 Hours after, perceived the fame Spot not 
fenhbly removed ; from which he concluded, the Pe- 
riod of its Revolution could not be fo (hort as one Day, 
(ince, if it were fo, the Change of Place of the Spot 
muft have been very fenhble in that Time. It were 
to be wifhed the Author had had Opportunities of con- 
firming this Period by more Obfervations, efpecially 
(ince it was necelfary to begin them foon after Sun- 
fet, and continue them till Venus was near the Hori- 
zon j the Strength of the Twilight in the firft Cafe, 
and the Thicknefs of the Atmofphere through which 
the Planet muft be feen in the latter, rendring the 
Obfervations very difficult. 
The 
