Proceedings of the Folytechxic AssociATioy. 855 



prism, or by observing tbe rays projected by a prism on a surface. 

 Thus all the prismatic colors are displayed at once in small patches, 

 and it is not easy to observe the shades of the prismatic colors, 

 because they are interfered with by the secondaries. Now, by the fol- 

 lowing method the rays may be, as it were, separated, and only one 

 color seen at a time, that too in large quantity. If, then, instead of 

 looking at the colors through a prism, the prismatic colors be thrown, 

 one at a time, from a prism in the hands of another party, dii'ectly 

 into the pupil of the spectator's eye, he sees the whole apartment in 

 one perfect blaze of each color in succession, and the effect is most 

 magnificent, even with a small prism, and when the sunlight is not 

 very strong. 



Marine Velocipedes. 



The revival of the velocipede, which was introduced into Europe 

 during the last century, merely as a source of amusement to pedes- 

 trians, has prompted a Frenchman to adapt one to be used on water 

 instead of land. It is now on Lake Enghein, twelve miles from 

 Paris, and is described as something like two snow shoes, held about 

 a yard apart by means of iron rods, between which is a propelling 

 wheel, about three feet in diameter, having paddles four inches wide 

 and eight long. It will be seen at once that this is a miniature repre- 

 sentation of the famous Burden steamboat, which appeared on the 

 Hudson river more than thirty years ago, and that it bears no resem- 

 blance to the land velocipede. The latter adapted to water would be 

 more like an Ohio river steamboat, with a wheel in front as well as 

 behind. Ex:[)eriments like the above have been repeatedly tried in 

 this country, and it is well settled that Imuran strength is most eco- 

 nomically applied to propulsion of a small boat by means of oars. 



After the reading of this item. Dr. Van der Weyde remarked that 

 some time since he had seen a marine apparatus which consisted of 

 a cylindrical hollow paddle-wheel, the interior of which was of such 

 , size that a man could stand upright therein and roll the thing through 

 the water by treading upon the interior surface, the wheel rotating 

 somethino^ after the manner of a treadmill. 



A gentleman said that this form was bad in its manner of dis- 

 placing the water as it advanced. It could not, of course, compete 

 with vessels of the usual form. 



Mr. Emery said that Scott Eussell had discovered that when canal 

 boat^J are towed above a certaiii speed the boats were found to be raised 



