Royal Society; 225 
for feveral weeks in the Atedlterranean j when the fun fhinos 
bright upon the water, fometimcs the upper part of the wave 
appears purpJifh, fometimes reddifh, tho' in fliallows, but it 
may receive this latter tin6ture from the fands which lie under 
it. When the wind has frefhencd, and the fliip has been under 
full fail, Dr. Smith obferved the waves at the head and fides 
of the /liip, to appear of a pale kind of brightnefs. 
Sailing towards the weft of Portland, we faw feveral por- 
pus's playing, with their heads above water , we alfo obferved 
a great part of the ^ropontis covered with them 5 and of thefe, 
Julius Solims is to be underftood, and not of dolphins, the' 
he ufes the word ^Delpbin-^ for no dolphins are caught in thofe 
feas by the Greeks, whole poverty, added to their love for 
fifii, and the advantage arifing therefrom, on the account of 
their fulemn fails and abftinences from all fort of flefh, has 
made them excellent fifhermen. 
An Error of common Surveyors, /;/ comparing Surveys taken 
at long Intervals of ^ime ivith the magnetic Needle, de- 
monftratedj by Mr. Will. Molyneux. Phil. Tranf N"" 230. 
p. (J25. 
THE variation of the magnetic needle is fo commonly 
known, that the explication thereof need not much be 
infifted on j it is certain, that the true folar meridian, and the 
meridian ihewn by a needle, agree but in very few places of 
the world ; and that too, but for a little time, the difference 
between the true and fnagnetic meridians perpetually varying, 
and changing in all places, and at all times j Ibmetimes to the 
caftward, and fometimes to the weft ward. On which account 
it is impoflible to compare two furveys of the fame place, taken 
at diftant times by magnetic inftruments, without due allow- 
ance be made for this variation , but this is not to be under- 
ftood, as if a map of the lame place, taken by magnetic inftru- 
ments at never lb diftant times, Ihould not at one rime give 
the fame figure and conients, as at another time 5 this 
certainly it will do moft exactly, the variation of the needle 
in no manner affecting either the ihape or contents of the 
furvey j all that is influenced thereby is the bearings of the 
lines run by the chain, and the boundaries between neighbours 5 
and how this may caufe a confiderable error, unlefs due 
allowance be made for it, is fully proved thus; let A, B, 
Fig. 2. Plate Vll. reprefent the liirvey of two lands, one in the 
pofJTeffion of A, and the other in the poireffion of B, taken, for 
Vol. III. F f inlhnce, 
