PROOF SPIRIT. 



sriltlT. A mixture of equal part* of water and 

 U woorally *> termed, but legally proof (pint is ,Ui.n..l l.y act of 

 parliament to be " such as shall, at the temperature of 51* Fahi . . 

 exactly Hths of an equal measure of distilled water." Such a n 

 consist* by weight of 50*76 water, and 49'24 alcohol ; and its specific 

 gravity it -930. [ALCOHOL.] 



PROl'ELLER. In works upon the construction of machinery the 

 word proftUtr it used to indicate the arrangement by meant of which 

 motion is given to a carriage bearing a portion of the working gear 

 required to traverte regularly in a horizontal direction ; or to indicate 

 a peculiar mechanism, act in motion by some mechanical power in 

 vceseU or ships which causes the Utter to advance by the resist- 

 ance offered by the water. The former clan of propellers are moat 

 frequently employed in tools, and metal-working machinery, and con- 

 sist of a fixed abaft bearing a screw thread, gearing into a nut work- 

 ing upon guides, so as only to be able to move laterally. The latter 

 an> more numerous, and may be said to consist of oars, sails, paddle- 

 wheels, and screw-propellers. Sometimes the locomotive engine is used 

 to propel loads on land ; but this is so manifestly a modification of 

 it* tractive powers that it may for the present at least be neglected. 

 Attention will in the following .article be principally called to the 

 propellers used for steam navigation, unquestionably the most impor- 

 tant machines of this description now in use. 



Until the application of the steam engine to the purposes of naviga- 

 tion, iurs and sails were the only propellers used. Oars are, in fact, 

 levers worked by band against a fulcrum fixed on the boat itself, and 

 causing the Utter to advance by the reaction of the water upon the 

 boat. Sails are propellers by reason of the resistance they offer to the 

 movement of the wind, which thus exercises a motive power upon the 

 vessel by it* transmission to the hull, through the masts. Accordingly, 

 therefore, as the sails are fixed with regard to the longitudinal axis 

 of the ship, do they cause it to advance either in the line of the keel, 

 to drop to leeward, or to turn on its vertical axis, with a velocity 

 depending upon the size of the soils, the velocity of the movement 

 of the wind, and upon the shape of the vessel These various condi- 

 tions come more especially under the province of ship-building, or 

 of navigation, and are therefore only alluded to here incidentally. 



The use of wheels bearing floats working in the water by the side of 

 the boats to which they were fixed, had been known from a very 

 remote period ; but it was not until the steam engine was applied as a 

 motive power to them that they were commonly introduced as pro- 

 pellers. At the present day the majority of steam-boats designed to 

 work in comparatively still water, and even the majority of the vessels 

 used in the deep-sea packet service, are provided with these organs of 

 locomotion. It is found, indeed, that paddle-wheel steamers can attain 

 a greater economical velocity than screw steamers hitherto have done ; 

 and the noise and vibration of the screw renders the class of vessel to 

 which it is applied so uncomfortable, that it cannot be considered applic- 

 able to passenger traffic. Upon some rivers and large artificial canals, 

 the waves caused by paddle-wheels are thought to be so injurious to 

 the banks, that screw-propellers are substituted for them ; but the 

 agitation of the water below the surface produced by the revolution of 

 the screw seems to be nearly as mischievous as the surface waves caused 

 by the paddles; so that the most important advantage possessed by 

 the screw propeller over the paddle-wheel, for narrow water-courses, 

 finally resolves itself into the smaller width it allows to be given to the 

 jveaseL In deep-sea-going vessels, which do not carry passengers, the 

 advantages of the screw are so great that nearly all the new traffic 

 'steamers are fitted with propellers of that description; and the 

 company founded by Mr. Smith, in 1836, which built the Archimedes, 

 has indeed already changed the whole condition of our commercial 

 marine. 



\Ylu-n paddle- wheels are used as propellers, they ore placed upon a 

 horizontal shaft or axis, and in front of the centre of gravity of the 

 vessel ; but there does not yet seem to be any uniformity of practice 

 amongst constructors with regard to the precise position of those parts 

 of the machinery ; and even in the case of some of the canal boats 

 used in Belgium the wheels are fixed at the stern, without any 

 apparently disadvantageous remits. In sea-going boats the wheels are, 

 however, always placed on the outside, and about the middle of the 

 boat ; they are keyed upon a horizontal shaft bearing upon the solid 

 framework of the ship's side, and almost always upon on external seat- 

 ing, carried by the framework of the paddle-box. The paddle-wheel 

 abaft is continuous through the vessel when only one engine is used, 

 and it is made in two pieces, with a movable clutch or coupling joint, 

 when two engines are required, as is the case in all sea-going vessels. 

 Motion U communicated to the shafts, either by means of cranks con- 

 nected with the ends of balance beams, or with the piston heads of 

 direct acting, or of oscillating, engines ; and the position of the cranks 

 toward* one another is made such that the piston of one engine shall 

 be able to exercise it* full power, when the other is over its dead 

 point*. The whecU themselves coiwist of a series of radial arms, con- 

 nected with one another by means of concentric rings (both the arms 

 and circles being made of wrought iron), an<l u]...n the arms are 

 fastened the floats, or the boards, whose revolution in the water pro- 

 duces the propulsive action. Very warm rli*cussioni have taken 

 WHO respect to the loss of power whi<-h necessarily attends the 

 ordinary form of wheel, in which the floats are fixed radially to 



PROPKT-T.ER ta 



the wliwl ; find many ingenious contrivances have been propoeed 

 IKWO of causing the floats to enter and to leave the 

 water in a ]>erpendie\ilar direction. For vessels making short runs, 

 the feathering paddles appear to produce satiafactory results; but it 

 seems to be more than questionable wluUu-r the runiplication of 

 the machinery, the additional weight of the wheels, ami the con- 

 sequent increase of power required to move them, do not more than 

 compensate for the theoretical advantage gained by diminishing the 

 lip observable in ordinary paddles. It is to be observed also, that 

 numerous modifications of the form of the float board* have been 

 introduced in order to diminish their inconveniences; and I 

 wheel, in which the boards are divided into narrow slipe, arranged in 

 cycloidal curves, so as to enter the water at the same place in 

 immediate succession, seems to be the best of its kind. Morgan's 

 feathering wheel is one of the most successful of those engines; 

 but after all the effective gain is very small, even with it. Gene- 

 rally speaking, the proportion of the useful efl'ect of paddle-wheel 

 engines to the power exerted is considered to be as 0'6 to 1-0 in 

 the case of radial paddles ; in the case of the feathering paddles it 

 hardly exceeds 0-66 to 1*00, although at times it may rise to 0-S33 to 

 1-000. 



Whatever description of wheel be used, there is a point on its radii 

 which describes a circle, known as the nlling circle, equal in its de- 

 veloped length to the distance actually traversed by the vessel during 

 one stroke of the piston, and which serves to measure the ratio of the 

 useful effect to the power exercised. It is important that the floats 

 should be placed beyond this line, and in sea-going vessels exposed to 

 cross, or rolling seas, the inner edge of the floats should even be kept 

 at some considerable distance beyond it. There is still a remarkable 

 discrepancy in the proportions assigned by different constructors to the 

 various details of paddle-wheels, but the most generally received ones 

 seem to be as follows. The diameter of the wheels is made equal to 

 about from 4 to 44 times the length of the stroke, in ordinary con- 

 densing engines of vessels designed for deep-sea sailing ; the breadth 

 of the wheel is made from J to J of the diameter ; the width of the 

 floats is made about ^ of the diameter of the wheel ; and their dis- 

 tance apart is made about 3 feet to 3 feet 6 inches. Some constructors 

 make the surface of their floats dependent upon the area of the mid- 

 ship section ; and its proportion then varies between ^ and fa of the 

 latter, the best proportion seeming to be about ^ of the said section. 

 The best velocity to be given to the outer edges of the floats appears to 

 be | of that of the horizontal velocity of the vessel. Wheels of large 

 diameter may be used on rivers without inconvenience ; but in sea- 

 going vessels a practical limit is placed to the increase of this dimen- 

 sion by the effect of the rolling of the ship, and the wheels are rarely 

 made more than J greater in diameter than the depth of the hold ; but 

 it must be repeated that at present all these rules are little better 

 than empirical ones. 



The screw has been occasionally used as a propeller for many years, 

 but it was not until 1836, as above stated, that its use became suffi- 

 ciently understood to allow of its being applied commercially. In that 

 year Mr. Smith attracted attention seriously to the subject ; but Cap- 

 tain Ericsson may fairly be considered to be entitled to the merit of 

 having been the first to overcome the practical difficulties connected 

 with the application of this description of propeller. At the present 

 day it certainly seems that for all ordinary deep sea goods traffic, and, 

 the experience of the Dutch also shows, for steam transport on canals 

 and narrow rivers, this description of mechanism will ultimately super- 

 sede every other one used for the same purpose. For vessels of war 

 the sheltered position of the screw, and of the engines, give that 

 description of propeller an incomparable advantage over the exposed 

 engines and wheels of paddle-boats. 



The screw-propeller ordinarily used may be described as consisting of 

 two or more arms (though at the present day it is very rarely that 

 more than three arms are used), forming parts of screws of large 

 diameter arranged upon a central shaft. This shaft is set in motion, 

 either by means of gearing, or by direct-acting engines working upon 

 the crank of the shaft ; so that the screw, being set in revolution, 

 advances in the water in the same manner that a wood screw would 

 advance in that material. The width of the screw is usually made 

 equal to about 4 of the convolution of the thread in small screws, and 

 to nearly J of that dimension in larger ones, whilst the pitch is found 

 to bo of the most advantageous proportion when it bears to the 

 diameter the ratios of 1*2 to 1 in large screws, and of 1'4 to 1 in small 

 ones. All these dimensions, however, must vary with the peculiar 

 form of the ship, and its resistance to motion in the water ; and in 

 Bourne's ' Catechism of the Steam Engine,' or, better still, in his 

 ' Treatise on the Screw-Propeller,' the several conditions affecting the 

 proportions of the screw are carefully discussed. The screw is usually 

 placed in the dead wood of the ship, immediately before the rudder ; 

 and its efficiency materially depends on the depth of iU immersion ; as 

 also upon the fineness of the stern of the vessel, perhaps almost as 

 much as upon the entry, or upon the midship section. The velocity 

 of revolution of the screw seems to range between 50 and 70 revolu- 

 tions per minute, according to the shape of the vessel and to the 

 dimensions of the screw itself. The size of the vessel to which the 

 screw-propeller is applied seems to have a more favourable influence 

 upon the useful effect produced, than is the case when the paddle- 



