285 



blue, as imagined by, Brewster, but red, green, and violet; the 

 first and last forming the terminal parts of the spectrum, and the 

 green occupying an intermediate position ; and the various tints 

 which intervene being the result of superpositions, in various quan- 

 tities, of these respecthe primary colours. He pursues the conse- 

 quences of this hvpothesis, applying it to a great variety of forms of 

 experiment, not only by the direct observation of beams of refracted 

 light, but by viewing the prismatic spectrum through different media, 

 capable of absorbing each of the primitive colours in different degrees : 

 and he finds the results to accord exactly with the hypothesis he pro- 

 poses, and on which he therefore concludes that their true explanation 

 must be founded. He conceives that the errors of preceding experi- 

 mentalists have arisen from their neglecting to take into account the 

 effects of diffraction, which introduces considerable confusion into the 

 results. 



A paper was also read, entitled, " An Investigation of the Laws 

 which govern the Motion of Steam- Vessels, deduced from expe- 

 riment." By P. W. Barlow, Esq. Civil Engineer. Communicated 

 by Dr. Roget, Sec. R.S. 



The author commences with the description of a paddle-wheel for 

 steam-vessels, of a new construction, in which the floats are made to 

 enter and leave the water nearly in a vertical position. He then inves- 

 tigates several formulae adapted to the calculation of the forces and 

 velocities arising from this form of the apparatus ; and gives an ac- 

 count of the results of various experiments made on its efficiency as 

 compared with the common wheels, and with relation to the con- 

 sumption of fuel. The general results to w hich he is led are as fol- 

 low : — 1st. When vessels are so laden as that the wheel is but 

 slightly immersed, little advantage is derived from the vertically acting 

 paddles. 2ndly. In cases of deep immersion., the latter has consi- 

 derable advantage over the wheel of the usual construction. 3rdlv. In 

 the common wheel, while the paddle passes through the lower portion 

 of the arc, that is when its position is vertical, it not only affords 

 less resistance to the engine, but is less effective in propelling the 

 vessel than in any part of its revolution. 4 thly . The paddle of the 

 wheel, while passing through the lower portion of the arc, affords 

 more resistance to the engine, and is more effective in propelling the 

 vessel, than in any part of its revolution ; a property which is a serious 

 deduction from its value: for, in consequence of the total resistance 

 to all the paddles being so much less than in the common wheel, 

 much greater velocity is required to obtain the requisite pressure, 

 and a greater expenditure of steam power is incurred. This loss of 

 power is most sensible when the wheel is slightly immersed: but in 

 cases of deep immersion the vertical paddle has greatly the advantage, 

 othly. In any wheel, the larger the paddles the less is the loss of 

 force ; because the velocity of the wheel is not required to exceed that 

 of the vessel in so great a degree, in order to acquire the resistance 

 necessary to propel the vessel. Sthly. With the same boat and the 

 same wheel no advantage is gained by reducing the paddle so as to 



