[ J74 ] 
difference of fpecific gravity were thought to be 
too fmall to be perceivable, till after the principal 
experiments were made, and their effe&s compared ; 
from which, as well as fucceeding experiments, thofe 
variations were found to be capable of producing a 
fenfible, tho’ no very confiderable effedt: however, 
as all the experiments were tried in the fummer fea- 
fon, in the day-time, and under cover ; we may fup- 
pofe that the principal fource of error would arife 
from the different weight of the column of the atmo- 
fphere at different times ; but as this feldom varies 
above -1-7 part of the whole, we may conclude, that 
tho’ many of the irregularities contained in the experi- 
ments referred to in the foregoing effayy might arife 
from this caufe; yet as all the principal conclufions 
are drawn from the medium of a confiderable num- 
ber, many whereof were made at different times, it 
is prefilmed that they will nearly agree with the 
truth, and be altogether fufficient for regulating the 
practical conftrudtion of thofe kind of machines, for 
which ufe they were principally intended* 
XX. An 
