A New Century of Inventions. James White
“Lieutenant S.’s blocks were reeved with a 21⁄2 inch double rope, and the common block with a 41⁄2 inch single rope, and both tackles suspended from a beam, and their respective falls let over the single blocks, so as to keep the weight applied as a power, just clear of the weight to be lifted, thus forming a power of six to one; the following experiments were made:
| Weight very slowly lifted. | Power required with Lieutenant S.’s blocks. | Power required with the common blocks. |
|---|---|---|
| ℔s. | ℔s. | ℔s. |
| 336 | 88 | 124. |
| 672 | 169 | 252. |
| 1344 | 312 | 448. |
| 2688 | 588 | 808. |
| 5376 | 1101 | 1344. |
“After reeving the common blocks with a 31⁄2 inch rope in lieu of a 41⁄2 inch rope, it was as follows: 5376 1101 1232.
“It must be observed, that the double 21⁄2 inch rope in Lieutenant S.’s blocks, is not of equal strength with the single 41⁄2 inch rope first used in the common blocks; and that his blocks had an undue advantage in the first experiment over the common blocks, in respect to the pliability of the rope. The rope should therefore, be taken larger in the one or smaller in the other case, on this account: The common blocks were reeved in the last experiment, with a 31⁄2 inch rope, which is as near as may be of the same strength as the double 21⁄2 inch rope.
“In these experiments it was observable, that the tar was much more squeezed out of the parts of the rope that passed over the smallest sheeves in Lieutenant S.’s blocks, than out of those passing over the larger sheeves, or out of those passing over the sheeves of the common blocks; by which, as well as by the nature of the thing, we judge that with blocks requiring such small sheeves, the ropes would be more crippled and broken than by the common blocks, especially if any constant strain or weight in motion, as on ship board, should be held by them. In regard to our opinion of the merits of the blocks proposed by Lieutenant S. compared with common blocks, we beg leave to submit, that the mechanical principle of them is very inviting, and it is not to be wondered that an ingenious person should pursue the idea; yet allowing there would be a saving of power, which is attained in so great a degree with the common blocks, but considering the greater complication, weight, and expence of these blocks, and their greater disposition to cripple the ropes, we do not perceive any application of them on ship board, for which we could recommend them in preference to common blocks; neither do we perceive any purposes on shore, for the services of the dock yards in which to recommend their application in preference to the other powers in use.”
To this account of the result of these experiments, I beg leave to add what seems to be a great improvement of this System: namely, a method by which the diameters of the larger pulleys are considerably lessened; and thus the principal, if not the only objection, obviated. It has been before observed, that the larger pulleys, as Q R, are the ultimate terms of an arithmetical progression, beginning at unity; and that consequently they cannot be very small, even though the first terms should be so. If a first pulley were only one inch in diameter, the twelfth pulley would be twelve inches—where we see a large and inconvenient difference. But this evil I now obviate, by placing at the beginning of the series, one or more loose pulleys, over which to reeve the cord, before the concentric or fixed grooves begin; thus lowering the ratio of the progression, and keeping the larger pulleys within bounds. For example, the smallest fixed pulley (supposed as before, to be one inch in diameter) I now make the second of the series instead of the first: and therefore, the second fixed pulley is to the first as 3 to 2, instead of being as 2 to 1; for the same reason, the third fixed pulley is to the second as 4 to 3; and in a system of 12 pulleys, (with one loose one) the respective terms will be as follows:
| Terms | 1 | — | 2 | — | 3 | — | 4 | — | 5 | — | 6 | — | 7 | — | 8 | — | 9 | — | 10 | — | 11 | — | 12 |
| loose; | 2⁄2; | 3⁄2; | 4⁄2; | 5⁄2; | 6⁄2; | 7⁄2; | 8⁄2; | 9⁄2; | 10⁄2; | 11⁄2; | 12⁄2 |
or 6 inches for the