144
A VOYAGE TO
[South Coast.
1802.
February.
wood at such times as the pits might require to be left for replenishing.
The necessary duties being all set forward under the superintendance of proper officers, I employed the following days in surveying and sounding. The direction of the port was too remote from the meridian to obtain a base line from differences of latitude, which, when observed in an artificial horizon, and at stations wide apart, I consider to be the best; nor was there any convenient beach or open place where a base line could be measured. It was therefore attempted in the following manner: Having left orders onboard the ship to fire three guns at given times, I went to the south-east end of Boston Island, with a pendulum made to swing half seconds. It was a musket ball slung with twine, and measured 9,8 inches, from the fixed end of the twine to the centre of the ball. From the instant that the flash of the first gun was perceived, to the time of hearing the report, I counted eighty-five vibrations of the pendulum, and the same with two succeeding guns; whence the length of the base was deduced to be 8,01 geographic miles.[1] A principal station in the survey of Port Lincoln was a hill on the north side, called Northside Hill, which afforded a view extending to Sleaford Mere and Bay, and as far as Cape Wiles on one side, and to the hills at the- ↑ This length was founded on the supposition, that sound travels at the rate of 1142 feet in a second of time, and that 6060 feet make a geographic mile. A base of 15′ 24″ of latitude was afterwards obtained from observations in an artificial horizon, and of 25′ 17″ of longitude from the time keepers with new rates, both correct, as I believe, to a few seconds. From this long base and theodolite bearings, the first base appeared to be somewhat too short; for they gave it 8,22 instead of 8,01 miles. The length of the pendulum in the first measurement was such as to swing half seconds in England; and I had not thought it, in this case, worth attention, that by the laws of gravity and the oblate spheroid, the pendulum would not swing so quick in the latitude of 35°. I must leave it to better mathematicians to determine from the data and the true length of a geographic mile in this latitude, whether the base ought to have been 8,22 as given by the observations and bearings: it was proved to be sufficiently near for all the purposes of a common nautical survey.
