Uoyal Society. 457 



On the other side of the wooden frame to which this thermometer is 

 fixed, a siphon barometer is attached ; and both these instruments are 

 made to act on iron-floats suspended by a thread, and counterpoised 

 over a pulley. Transverse wires are affixed to these threads, and are 

 forced against a sheet of ruled paper on a frefme, which from its 

 connexion with a clock is advanced a certain space each day. by a 

 spring hammer forming part of the striking machinery of the clock. 



" On the action of light upon the colour of the River Sponge." 

 By John Hogg. M.A., F.L.S., C.P.S., &c., Fellow of St. Peter's Col- 

 lege, Cambridge. Communicated by Thomas Bell, Esq., F.R.S. 



The author found that the green colour of the Spongilla fluviatilis, 

 or river sponge, is acquired solely through the agency of light, and 

 is lost when the sponge is removed from its influence. As this does 

 not appear to be the case with Actiniee, the Hydra viridis, or any 

 other Polype, the author is disposed to consider this production as 

 being nearer allied to the Algse or Fungi, than to any tribe belonging 

 to the animal kingdom*. 



" Researches on the Tides. Ninth Series. On the deduction of 

 the Laws of the Tides, from short Series of Observations." By the 

 Rev. W. Whewell, M.A. Trin. Coll., Cambridgef. 



It is very desirable to ascertain whether it is possible to deduce 

 the laws of the tides from short series of observations ; since, if it 

 be so, not only does the construction of good tide tables for diflfer- 

 ent places become more easy ; but also the value of tide tables is 

 much increased, if the predicted tides agree with those of each year 

 as' well as with the mean of many years. The object of the author 

 in the present paper is to determine this point by the discussion of 

 several years' observations of the tides at Plymouth and at Bristol. 

 The calculations for the former place were executed by Mr. Dessiou 

 and Mr. Ross in the Hydrographer's Office at the Admiralty; the cal- 

 culations for Bristol were performed by Mr. Bunt, in virtue of a 

 grant of money from the British Association. The result of these 

 discussions is, that a very regular form and good approximation for 

 the semimenstrual inequality may be obtained from the observations 

 of one year ; that the existence of the lunar parallax corrections ap- 

 pears very clearly in the observations of one year ; and that its value 

 may be determined from a series of three or four years. The lunar 

 declination corrections are more irregularly given by short series of 

 observations ; but in a series of four or five years, the general form 

 and approximate value of the corrections become manifest. In the 

 course of these calculations such questions as the following were 

 proposed, and their solution attempted : 1 . To which transit of the 

 moon ought we to refer the tide ? It appears that the transit which 

 produces the best accordance with theory, is that which Mr. Lubbock 

 terms transit B, which is an epoch about 42 hours anterior to the 

 high water at Bristol and Plymouth. 2. How does a change of 



* [Mr. Gray arrived at the same conclusion, but on physiological grounds, 

 some years since. See Zoological Journal, vol. i. p. 50. — Edit.] 

 t See Load, and Edinb. Pliil. Mag. vol. xii. p. 334, note. 



