284

Rotary Pumps and Steam-engines.

[Book III.

formed is wound round the spindle. In 1530 Jürgen of Brunswick de-vised a machine which dispensed with this intermitting action of the fingers,i. e. he invented the spinning wheel, which rendered the Operation oftwisting the filaments uniform. The wheel, however, like the primitiveapparatus it was designed to supersede, produced only one thread at atime; but in the last Century Hargreaves produced the “ spinning jenny,”by which a single person on turning a wheel, could spin eighty-four threadsat once; then followed the “ rollers” of Arkwright, the “ mule” of Crompton,to which may be added the “ gin” of Whitney, and also “ carding engines,”in place of the old hand cards, all composed of and put in motion byrevolving machinery :—these have indefinitely extended the spinning ofthread, and relieved females from a species of labor that, more than anyother, occupied their attention from the beginning of the World; and lastly“ power looms” impelled by water, wind, steam, or animals (through theagency of circular movements) are rapidly superseding the irregulär andalternating motion of human hands in throwing the shuttle to and fro.

The conversion of intermitting into continuous circular movements isalso obvious in ancient devices for raising water. The alternate action ofthe swape, the jantu and vibrating gutter thus became uninterruptedin the noria and tympanum—the irregulär movement of the cord andbücket became uniform in the chain and pots; and so did the motion ofthe pitcher or pail as used by hand, when suspended to the rim of a Per-sian wheel. And when the construction of machines did not allow of asuitable change or form, they were often worked by cranks or other simi-lar movements ; (of this several examples are given in preceding pages;)but in no branch of the arts has this preference for circular movementsover straight ones been so signally exhibited as in the numerous rotarypumps and steam engines that have been and still are brought forward;and in no department of machinery has less success attended the change.Most of these machines that have hitherto been made may be consideredas failures; this result is consequent on the practical difficulties attendingtheir construction, and the rendering of them durable. These difficulties(which will appear in the sequel) are to a certain extent unavoidable, sothat the prospect of superseding cylindrical pumps and steam-engines isprobably as remote as ever.

At an early stage in the progress of the machines last named, it becamea desideratum with engineers to obtain a continuous rotary movement ofthe piston rod in place of the ordinary rectilinear and reciprocating one,that the huge “ walking beam,” crank and connecting shaft might be dis-pensed with, the massive fly-wheel either greatly reduced or abandoned,and the power saved that was consumed in overcoming their inertia andfriction at every stroke of the piston. Reasoning analogous to this hadlong before led sonne old mechanicians to convert the motion of the com-mon pump rod into a circular one ; in other words, to invent rotary orrotatory pumps. By these the power expended in constantly bringingall the water in the cylinder and suction pipe, alternately to a state of restand motion was saved, because the liquid is kept in constant motion inpassing through them. The steam engine was not only originally de-designed as a Substitute for pumps to raise water, but in all the variety ofits forms and modifications it has retained the same analogy to pumps asthese have to bellows. One of the oldest of modern rotary steam engines,that of Murdoch, was a copy of an old pump, a figure of which, No. 131.is taken from Serviere’s Collection.

Two cog wheels, the teeth of which are fitted to work accurately intoeach other, are enclosed in an elliptical case. The sides of these wheels