Page:Proceedings of the Royal Society of London Vol 4.djvu/466

Rh either in the cellular tissue, or in the minute blood-vessels of the iris. It is from elasticity, he believes, that the iris derives its power of accommodation to changes of size, and its tendency to return to its natural state from extremes, either of dilatation or of contraction ; but beyond this, elasticity is not concerned in its movements.

The following papers were read, viz. —

1. "Tide-Observations at Tahiti:" by Captain Edward Belcher, R.N. Communicated by Captain Beaufort, R.N., F.R.S., &c.

This paper consists of copies of the Tide Journal, registered at the Island of Motuatu, in the Harbour of Papeete, and of a short comparative series made at Point Venus. They were conducted by Mr. M'^Kinley Richardson, Mate. The construction of the tide- gauge is described ; and an account is given of the methods of ob- servation, and of the precautions adopted to ensure accuracy. The results are specified in the following letter from the author to Cap- tain Beaufort, which accompanies the paper : —

" Her Majesty's Ship Sulphur, Woolwich, August 2, 1842. " SiR,-~Referring to the Tide Registries, forwarded on my arri- val, I beg leave to offer the following general remarks upon the tides at Tahiti.

In consequence of your very special instructions relative to the determination of the actual periods of high water at the Island of Tahiti, the most minute attention was paid to this subject ; and as these periods could only be approxhnated, recourse was had to my old method (successfully practised in the Lancashire survey), of de- riving them from the Equal-altitude system.

" By a reference to the Tide Registry annexed, it will be found that there are two distinct periods of high water ^ during each interval of twenty-four hours ; and that during the seven days preceding, and seven days following the full and change, they are confined be- tween the limits of 10 a.m. and 2^ 30™ p.m., the whole range of interval, by day as well as by night, being about ^ 27"^.

" Commencing with the seventh day preceding the full moon, viz. the 9th of April, it will be perceived that high water occurs at 10 A.M., this being the greatest a.m. interval from noon; and that on the 16th, at the full moon, it occurs nearly at noon.

" Passing on to the 23rd, it reaches the greatest p.m. limit at 2'^ 30"^, and on the 2nd of May again reaches the noon period.

" Between the 23rd and 24th, however, a sudden anomaly presents itself. Throughout the day of the 23rd, the variation of the level does not exceed 2^ inches, and the general motion is observed to be ' irregular.' The time of high water is also the extreme p.m. limit.