210 DESCRIPTION or a MACHINE, &e, 
for the admiffion of aglafs tube, of the fize of a common 
barometer tube, and cemented there. The fea water wall 
rife in the copper pipe to the general level of the fea, but 
will not appear in the glafs tube becaufe the copper pipe 
enters the thip above the water line, as before obferved,. 
But if a quantity of oil be poured down the glafs tube, the 
furface of the oil will rife and become vilible in the tube, 
‘on account of the fpecific difference between -oil and fea 
water. 
This glafs tube muft alfo be furnifhed with a fcale for 
meafuring the different heights of the oil, the cypher, 
or (0) of the feale being on aline with the furface of the oil 
when the fhip is at reft, or makes no way. But when 
fhe is ina progreflive motion, the water contained in the 
copper tube, together with the column of oil in the glafs 
tube, will be forced upward, in proportion to the velocity 
with which the veflel proceeds; which will be afcertain« 
ed by the different altitudes of the furface of oil, vilible 
on the graduated {cale. | 
The glafs tube fhould be made to run fome depth into 
the copper pipe, and alfo be of a fufficient height above, 
to allow room for the vibrations of the column’ of oil, 
when the fhip is agitated by the waves. 
When the fhip has got every thing on board and whilft 
fhe is under no way, the furface of the oil muft be regu- 
lated by bringing it even with the (0) or cypher of the 
feale; and this examination, fhould be frequently made 
on account of the confumption of provifions and other 
wafte, that may alter the fhip’s draught of water. 
In taking down the reckoning from the f{cale, the moft 
favourable moment fhould be watched for a fair obferva- 
tion, viz, when the fhip is proceeding with an average 
velocity, not when fhe is in the act of plunging into, or 
rifing 
