The pullies are in general arranged in their blocks, by being placed one by the fide of the other on their pin, or one directly under the other upon feparate pins; but in either of these modes an inconvenience arifes if more than three pullies are framed in one block. If, according to the first method, three pullies are placed by the fide of another, as the last line by which the draught is made, (or the fall of the tackle, as it is commonly called) muft neceffarily be upon the outfide pulley, the difference of their friction will give it fo great a tendency to draw the block awry, that as much will be loft by the rubbing of the pullies against the block on account of it's obliquity, as will be got by increafing the num ber of lines. The fecond method is free from this objection; but as the length of the two blocks taken together must be equal to the fum of the diameters of the fix pullies, befides the intervening spaces for the ropes, and the neceffary appendages of the framing, they would run out into fuch an inconvenient length, as to deduct very confiderably from the height to which the weight might otherwise have been raised. Hence it is evident that no very great purchafe can be obtained by the common tackles of pullies alone; to increafe it's power, a fecond tackle is fometimes fixed upon the fall of the first, but here the height to which the weight might have been raifed by the firft will be lefs in the fame proportion as the purchase is increased by the fecond. Thefe impediments are obviated by Mr. Smeaton's contrivance; the pullies are here (fig. 6, pl. 5.) placed in each block in two tier, feveral being placed upon the fame pin, as in the firft method; every one having another under it, as in the fecond. In this method all the lines are clear of one another, and the blocks are kept parallel. The model before you confifts of twenty pullies, five on each pin; with this model you may raise fix hundred weight; but with a tackle of this fort properly executed in large, one man will easily raise a ton. Twenty is the largest number of pullies that answers well in practice. The pullies are placed in two tier in each block, feveral being upon the fame pin as in the first method, and every one having another under it as in the fecond method; when the tackle is in ife, the two tier that are the most remote from cach other are fo much larger in diameter than those that are neareft, as to allow the lines of the former to go over the lines of the latter without rubbing. This conftruction gives a new method of reeving the line upon the fheeves; for here let the number of fheeves be what they may, the fall of the tackle will always be upon the middle fheeve, or that next the middle, according as the number of pullies on each pin is odd or even. To reeve it, let the line be fixed to fome convenient part of the upper block, and brought round the middle fheeve of the large tier of the under block; from thence round one of the fame fort next to the middle one of the upper block; and fo on, till the line comes to the outfide theeve, where the last line of the larger theeve falls upon the first fheeve of the smaller, and being reeved round thofe till A large tackle was tried on board one of his Majesty's fhips, and though the rope was new, one man raised a ship gun and carriage of 2700 weight, there being a person to hold on to prevent the rope from flipping. till it comes at the oppofite fide, the line from the laft fheeve of the fmaller tier again rifes to the first of the larger, whence it is conducted round till it ends on the middle fheeve of the upper block on the larger tier. OF A COMPOUND ENGINE. Fig. 1, pl. 5, reprefents acompound engine. The endless fcrew is turned round by the wheel A B, the screw takes into a wheel DC; on the axis of this wheel is a pinion E, that moves the wheel F G; to the axis H of this wheel the running rope of the pullies IK is fixed. The diameter of the fly-wheel or lever is twice that of the wheel CD; the wheel CD has 64 teeth, the pinion E on the axis of this wheel has 10 teeth, the wheel FG has 80 teeth. To calculate the force of this engine, multiply the diameter 2 of the fly by 64, the number of teeth in CD, which gives 128, and this product by 8, the number of times the pinion E moves round while the wheel FG moves once round, which gives 1024; multiply this by 3, because the diameter of the wheel FG is three times that of it's arbor H, and you obtain 3072; which multiplied by 4, the advantage gained by the pullies, gives 12288 to 1 for the power of this engine, or 768 pounds to one ounce. ABCDEF, fig. 2, pl. 6, is a smaller compound engine; the wheel or fly C has a fcrew on it's axis, which works into the teeth of the wheel D which has 48 teeth, and the diameters of these two wheels are equal; but the wheel C moves 4& times round while the wheel D is moving once round; but the diameter of the wheel D is 6 times that of it's arbor, on which the cord is coiled that raifes the weight; confequently the power is as 6 times 48 or 288 to one, or one ounce will fupport 288 ounces. On On the upper part of fig. 2, pl. 5, and fig. 2, pl. 4, is reprefented a compound engine, confifting of three levers acting one on the other; they are all of the firft kind. In the firft, R, the proportion is as 1 to 5; in the fecond S, as I to 4; in the thirdT, as 1 to 6; or as 1 to 5 X 4 X 6, equal to 120; confequently if the power be to the weight as I to 120, they will counterpoife each other. OF WHEEL WORK: Wheels with teeth are used in fuch a variety of ways, that it will be neceffary to explain in fome measure their nature and action. Although the computation of the numbers is not a part of mechanics, yet it is ufeful in the conftruction of a variety of engines. From the nature of the wheel and axle it is clear, that the power or force on the pinion is to that on the circumference of the wheel on the fame axis, as the diaineter of the wheel is to that of the pinion. Whether a wheel drives a pinion, or a pinion drives a wheel, the number of turns of the wheel multiplied by the number of it's teeth is equal to the number of turns that the pinion makes in the fame time multiplied by it's teeth; fo that the number of cotemporary turns of a wheel and pinion are reciprocally proportional to their number of teeth. Thus, if the number of teeth in a wheel be 80, and thofe in the pinion 8, then the pinion will turn round 10 times to once of the wheel; therefore the quotient of the wheel divided by the pinion it drives, is the ratio of turns to unity; thus gives 10. Wheels and pinions are therefore ge 80 8 nerally expreffed by fractions, the numerator being the wheel, the denominator the pinion. Hence if there be any number of wheels acting upon fo many contiguous pinions, and you divide the the product of the teeth in the wheels by thofe in the pinions, the quotient will be the number of turns of the laft pinion, in one turn of the first wheel. The number of turns may therefore be expreffed by a fraction, whofe numerator is the number of teeth in the leading wheel, multiplied by it's number of evolutions, and the denominator the number of teeth in the wheel that is driven. Thus a pinion of 6 acting on a wheel of 52, will make it turn 3 times while it turns 26 times itself equal 3. 6 x 26 52 If therefore you have a piece of wheel work, confifting of feveral wheels and pinions, ex. gr. if a wheel of 48 acts on a pinion of 8, on whofe axis there is a wheel of 40 taking into a pinion of 6 carrying a wheel of 36, which moves a wheel of 36, on whofe axis there is an index. The number of turns made by the index while the first wheel goes once round, will be expreffed by this fraction, 48 40 36 X X g 6 6 240. Hence it is evident that any number of teeth on the wheels and pinions, having the fame ratio, will give the fame number of revolutions to an axis 64 at a given distance from the first, thus xx 10 8 36 6 give 240 like the preceding combination; it is therefore left to the kill of the artift to determine what numbers will beft fuit the general defign and circumftances of his machine. Thus the fame motion may be performed either by one wheel and pinion, or many wheels and pinions, provided the number' of turns of all the wheels bear the fame proportion to all the pinions which that one wheel bears to it's pinion. Thefe principles conduct us to a rule, whereby Y the |