interruption to the course of the machine from the contrarious impact of the air, however quick or however slow the motions, either of the screw itself or of the machine which is propelled by its operation. In attestation of this fact and as showing the immunity of the perfect screw from the disparaging effects experienced by the other modes of accomplishing the same object, I will only mention a circumstance related to me by Mr. Smith himself, to whom I am glad to acknowledge myself indebted for so much valuable information respecting this instrument, which, by the light he has thrown upon its use and the improvements he has introduced into its construction, he may be truly said to have made his own. Upon a late occasion, when trying one of the larger class of vessels which had just been furnished by him upon this principle, some persons not perceiving the true nature of the figure employed, contended that some opposition must be experienced by the central portion of the screw, which revolved so much less rapidly than the rate of the ship itself. In order to convince them of their error, Mr. Smith caused a portion of the surface in question, next the axis, to a certain distance, to be cut away, leaving an opening, by which, for the water to escape. The result was, immediately the loss of one mile an hour in the rate of the ship; thus shewing that even the most apparently feeble portion of the impinging surface of this instrument contributes, in its degree, to the constitution of the aggregate force of which it is productive.