The mechanics' magazine, museum, register, journal, and gazette. No. 1128, Saturday, March 22, 1845 / edited by J.C. Robertson.

Date:
[1845]
Catalogue details

Licence: Public Domain Mark

Credit: The mechanics' magazine, museum, register, journal, and gazette. No. 1128, Saturday, March 22, 1845 / edited by J.C. Robertson. Source: Wellcome Collection.

    8/20 (page 198)
    we shall have one-third more surface liable to leakage than before, and con¬ siderably more than an increase of one- third in the amount of friction when pro¬ gressing on a level; but if we are as¬ cending an incline of 1 in 240, we must have twice the amount of adhesive re¬ sistance, and to obtain it six rollers every 30 feet; if an incline of 1 in 120, three times the adhesive resistance, and nine rollers every 30 feet; and if an incline of 1 in 80, no less than four times the adhesive resistance, and twelve rollers every 30 feet; and as a matter of con¬ sequence it follows, that in those parts of the line where the greatest propelling power is required, there will exist the greatest amount of surface liable to leak¬ age, and its power-destroying results. But this is not all: every roller must project more or less into the tube, and must of necessity render the passage of the piston far less air-tight than it would be if the interior of the tube presented a perfectly plain and unbroken surface from end to end. I differ from Mr. Cheverton in his opinion, that, in the use of cogged pinions and racks, leakage would be prevented on the piston passing the pinions, by the “closing up with a cog in the plane of the piston what otherwise would require to be left as an aperture,” for this reason, *—that, if the cogs could be made to fit so accurately as to prevent the passage of air, this would require a never-failing perfection of self-adjustment, between the racks and pinions, hopelessly unat¬ tainable, where some parts of the ma¬ chinery are stationary, and occasionally brought into sudden connection with other parts moving with a velocity of 36 ft. or more per second. And it appears to me, that the substitution of plain rollers for cogged pinions will but increase this serious evil; and though there may be more ways than one of lessening the difficulty, I think in practice it will be found that, every time the piston passes one of the projecting rollers, a gush of air will pass into the exhausted part of the tube. It is also obvious that the pivots of the rollers will be forcibly pressed against one side of their sockets; and as the uneven wear consequent upon this increases, the rollers will be sepa¬ rated, by the pressure of the piston bar, more widely than they should be, and the adhesive resistance proportionately decreased. . It is absolutely necessary, as Mr. Pil- brow, in his specification, shows, that the piston-rack and carriage-rack should maintain the same relative position ; that is to say, the piston-rack being pre¬ cisely under, and matching end to end with the carriage-rack, and, therefore, as long as the power applied continues, and the piston advances, the carriage will do the same to the end of the tube ; “neither arriving before or after the other, but together, as they cannot se¬ parate, nor can one move or stop without the other.” This may be done with cogged racks and pinions, but I ask Mr. Pilbrow, or your correspondent “ E. E.,” how the same thing is to be effected with plain rollers and bars ? How, if suffi¬ cient adhesion can thus be obtained for the ordinary purposes of traction, the additional adhesion necessary to prevent the piston-bar being separated from the carriage-bar, and vice versa, when any unusual strain or obstruction occurs, is to be secured without a most fearful loss of power, or the use of something very analogous to cogs ? The models publicly exhibited by Mr. Pilbrow', which work so satisfactorily (apparently), seem to have satisfied “ E E.” that “ the principle is perfectly sound, and its practicability not to be doubted;” but unfortunately, the more I investigate the subject, the stronger are my doubts, and I do not think they will be removed until I see the invention ;practically tested. Thanking you for the insertion of my former communications, I am, Sir, yours respectfully, D. N. March’14, 1845. THE ARTIFICIAL HAND-—BY SIR GEORGE CAYLEY, BART. [Continued from page 175.] In the structures before described a pressure or grasp between the thumb and fingers near their extremities is effected, and also the holding of substances of a moderate size near the middle of the hand; and, perhaps, this may in most cases be as much as is required w'here the person employing it has only lost one hand, and can, therefore, do any more difficult task with the other; but there are persons who have had the mis¬ fortune to lose both hands, in which