600 Foreign and Miscellaneous Intelligence. 



that where a jet of water is discharged under mercury, the results 

 are the same, under a given force, as when it takes place in water, 

 or air, the quantity discharged being in all cases the same, in the 

 same time. 



Hence, it appears that the force with which a moving or spouting 

 fluid recoils is not affected by the surrounding medium, however 

 rare or dense it may be : and thus we may understand why the 

 attempts, which have been made to propel vessels, by forcing water 

 through them against water, have not proved advantageous. 



The well known fact that large rivers penetrate, in a direct course, 

 far into the ocean, notwithstanding its agitation by tides and cur- 

 rents, is somewhat analogous ; and were it not for this remarkable 

 degree of mobility in water, the sediment, which is now mostly de- 

 posited at a considerable distance in the sea, would accumulate near 

 the mouths of rivers, and tend to divert them from their course. 



Whilst making my experiments on the jet of water, I noticed that 

 when sand was dropped into the water near the orifice from which 

 the jet issued, it was drawn laterally toward the hole, till it distinctly 

 appeared to enter it, but it was in fact only an optical deception, the 

 grains of sand being carried away by the jet as soon as they came 

 in contact with it, with such great velocity as to be perfectly invisible. 



5. OPTICAL DECEPTION UPON THE LIVERPOOL AND MANCHESTER 



RAIL ROAD, 



This rail road consists of two lines of rails, so that in looking from 

 the carriage window of one line, the other is seen, and presents the 

 following somewhat remarkable appearances. While travelling at 

 the rate of from 12 to 15 miles per hour, the rail, together with the 

 roadway, the banks, and other objects, appear, as they do from the 

 window of a stage-coach, to recede, or move in a direction the reverse 

 of that in which the carriage is moving. But when the speed in- 

 creases to twenty-four or thirty miles in the hour, the rails no longer 

 seem to recede, but to move with the carriage, as if they were run- 

 ning along the road at the same rate as the spectator. These dif- 

 ferent appearances, accompanying the different speeds, are explained 

 without difficulty ; they depend on the facts which are familiar to 

 every one who has caused a fluted pencil case, having a plain slider, 

 to revolve in the hand. The case is obviously seen to move, but the 

 slider seems stationary, because, being plain, it presents at every period 

 of its revolution precisely the same appearance to the eye. If the 

 iron rails appeared always to move along the road, the explanation 

 of the phenomenon would already have been given. The varying 

 effect produced by varying speeds depends on the circumstance that 

 the iron rails are not, like the slider of the pencil case, quite plain, 

 but have slight irregularities occurring at short intervals, which 

 with a moderate velocity are visible, and give to the rail a receding 

 appearance ; but when the velocity becomes doubled, the impression 

 on the retina produced by one irregularity is not effaced till it is 

 succeeded by others, so nearly similar, that the appearance of the 



