steam, and passing off with it, which was proved, not only by its taking into its current atoms of dust, but also at a distance of six or eight inches it would draw the flame of a candle from its wick. This current of air would of course cool down the steam, and that in proportion to its velocity. Knowing this fact, it is easy to understand the cause of the disk being pressed so closely against the aperture.

"The steam passes off horizontally in every direction, as M. Clement observes, in a star-like form. Now as a strong current of air will be created by the velocity of the steam, and pass off with it (vide page 214), the surrounding air will follow, and in its course, as in fig. 1, it will impinge on the disk and cause a pressure, and that in proportion to the height or velocity of the steam. When the safetyvalve is so constructed, which is sometimes the case, as to allow the steam to pass off in a horizontal direction, the pressure will be much greater than when the valve is conical, which gives a different direction to the steam, as at fig. 2, making it more difficult for the air to impinge on the valve. The valve, placed on the generator of the highpressure safety engine (Mr. Perkins's), is so small, and so nearly cylindrical, that the stream, when it issues from the valve, causes the molecules of steam immediately to join and form one volume, carrying the current of air upwards as the steam passes nearly perpendicularly. (See fig. 3). No air can, of course, come in contact with the valve; probably a partial vacuum would be made immediately over the valve. This evil may be very easily remedied by obliging

the steam to rush into a tube rising twelve or more inches above the valve. In fact, all steam-boats are provided with a steam-box, into which the steam rushes, which necessarily prevents the current of air from acting on the valve. The steam issuing from the safety-valve would then pass off perpendicularly, and prevent the admission of the atmospheric air, which created the pressure on the disk observed by M. Clement." Mr. Perkins concludes by saying, "Science is much indebted to M. Clement for his discovery; for although this phenomenon has been observed in strong jets of water rushing from a fountain, as well as in strong currents of air rushing from the blower of a blast furnace, yet M. Clement is undoubtedly the first who observed it in steam, and also the first who warned the public of the danger of using this kind of valve."*

*The above interesting fact may be illustrated by an apparatus so very simple, that any of our readers may try the experiment. Take a quarter of a sheet of letter-paper, roll it round a black-lead pencil, and with sealing-wax fix the outer loose edge; withdraw the pencil, and you have a paper tube. Cut a circular piece of card, about the size of a crown piece; make a hole in the centre, in which fix the paper tube, by dropping a little wax in the angle formed by the tube and the card. On the surface of the card, near the edge, in three equidistant places, put a little drop of wax, about 1-16th inch high. Cut a disk of card, rather smaller than the former, so as to lay freely within the three drops of wax on the fixed card. The section would then appear thus. Hold the tube, as in the figure, and support the disk against the card by the finger; and then blow forcibly through the tube, and withdraw the finger from the disk, and it will not fall down or be blown away; but the moment you leave off blowing it will drop. Disks of metal may be suspended in the same manner. These

MODERN ENGINES.

In the engines of Bolton and Watt, which have been already described, it will be observed that the great beam rests upon the wall of the building, which consequently has to bear a great part of the strain of the work. They now construct their engine so as to confine the strain of working, within its own framing. Engines of this construction are called "Portable," or, more properly, Independent Engines. The beam is made lighter, and in somo mali, engines is indeed so light, that it may be considered as merged in the parallel motion; and in some cases the beam is altogether dispensed with.

Fig. 50 will give a good idea of a well-constructed modern engine; it is from a drawing of a twelvehorse engine by Mr. W. Dryden. Mr. Maudslay, and other eminent engineers, adopt nearly the same plan, im engines of the same power, however they may differ in small engines. A beautiful twelvehor se engine, on this plan, was put up by Mr. audslay for Messrs. Pellatt and Green, at their extensive glass-works, near Blackfriars Bridge,

M

London.

MODERN ENGINE, AS CONSTRUCTED BY
BOLTON AND WATT, AND OTHERS.

A, A cast-iron cistern; B, B, B, cast-iron pillars, supporting the frame or entablature, c. On this frame

phenomena formed the subject of a very interesting lecture by Mr. Faraday at the Royal Institution.