xiv rhe P R E FA C E. 
cafl; from what we know, on the 
adjoining Borders of Terra incognita-, 
it feems therefore reafonable in fome 
degree to indulge Conjecture there ; 
otherwife we iTiould make but very, 
flow Advances in future Difcoveries, 
either by Experiments or Reafon- 
ing : For new Experiments ancl 
Difcoveries do ufually owe their firft 
Rife only to lucky Guelfes and pro- 
bable Conjectures, and even Difap- 
pointments in thefe Conjectures,' do 
often lead to the Thing fought for: 
Thus by obferving the Errors and 
Defects of a firft Experiment in any 
Rcfearches, we are fometimes car- 
ried to fuch fundamental Experi- 
ments, as lead to a large Series of 
many other ufeful Experiments and 
important Difcoveries. 
If therefore fome may be apt to 
think that - \ have fometimes too 
- ' far 
