438 REPORT — 1856. 



believe, about the average present practice in the highest class of our mer- 

 chant steam-shipping, and this measure of the unit would facilitate calcula- 

 tions; but whether 33,000, or 100,000, or 132,000, or any other number of 

 lbs. weight, raised 1 foot high per minute, be adopted as the statute unit of 

 horse-power, is a mere matter of convenience, a question very proper for 

 being submitted for the consideration and recommendation of a Committee. 



V^D^ V^xmid. sec. 



As to the question whether tiie formula -: — . , ^ or —■ — r-r- would 



' ind. h.p. ind. b.p. 



be the better formula for determining the relative dynamic merits of steam- 

 ships, these formulae are, as respects similar types of immersed form, a mere 

 transformation of terms, for in similar types of form the immersed midship 

 section will vary in the same proportion as the cube root of the square of the 

 displacement. These formulae would therefore give proportional results. I 



/ V D^ \ 

 have, however, preferred the formula based on displacement I -^—z — r-^— i, 



because this formula may, as I believe, be made the base of calculations as 

 to the capability of ships for mercantile service, for which purpose the for- 

 mula, based on midship section, without reference to displacement, is 

 inadequate. 



The Table of data now supplied by Mr. J. R. Napier, is a valuable addition 

 to our statistical data, in so far that, after having determined the relative 

 dynamic merits of the ships referred to, and classified them accordingly, the 

 information afforded by this Table will aid in the analysis of their respective 

 types of form. T would, however, beg to suggest that the position of the 

 centre of gravity of the immersed midship section of each ship, expressed by 

 its depth below the surface line, at which the displacement has been cal- 

 culated, would be a very important addition to this Table, and it is hoped that 

 Mr, James R. Napier will be able to supply it. 



In fact, it is in consequence of the depth of the centre of gravity not being 

 noticed in the formula above referred to, that I have spoken of it in my 

 paper as " open to criticism " and probable amendment, and I shall be pre- 

 pared in Committee to submit this view of the case for consideration. 

 Requesting that this letter be read conjointly with Mr. Napier's remarks, 

 I have the honour to be. Sir, your most obedient servant, 



Charles Atherton. 



On the Vital Powers of the Spongiadoe. 

 By J. S. BowERBANK, F.E.S., F.G.S. ^c. 



Inhalation and Exhalation. 

 Some years since I received from Dr. Lister of Madeira two masses of a 

 Halichondriaceous sponge, each about 7 inches in diameter, accompanied 

 by the information that they were portions of the same individual, and I 

 was struck by the remarkable difference in the external appearance of the 

 two. In one, the oscula were nearly all widely expanded, several of 

 the orifices being nearly half an inch in diameter; while in the other 

 piece scarcely any of them were open, the greater part being entirely closed 

 by a stout membranous veil, which in many cases was pursed up in the 

 centre of the area in a conical form. On writing to my friend to inquire 

 the cause of this difference in the appearance of the two pieces of the 

 same sponge, he informed me that the piece with the closed oscula had been 



