These are questions suggested by a letter from an esteemed correspondent, who has written us from Australia, in regard to his supposed discovery that pressure will perform work. He describes the method by which he intends to utilize the pressure of the sea upon the bottoms of floating vessels, to propel them without the aid of steam, so indefinitely that we can get no clear idea of the means employed. The questions, which his letter has suggested as a subject for the present article, are not new ; they have been long the subject of thought and discussion, and have been definitely answered. But though it is known to physicists,that pressure, merely, can never perform work, there are many men like our correspondent whose minds are not clear upon the subject,and errors in their views, and mistakes in invention, frequently arise from this cause. First,what is pressure ? A ball of rubber rests upon a table. It does not, regarded as a mass, move relatively to the point of support. If we examine this ball minutely, we shall find that its shape—supposed to be perfectly spherical when no pressure is sustained—is no longer a sphere, but has become a spheroid, whose shorter axis lies on a line drawn through its center and the center of the earth's attraction. If now the support be suddenly lowered away from the ball, pressure either is lessened or annihilated, and the ball, considered as a mass, begins to move; and we find by further experiment, that so long as the conditions of pressure are not altered, no relative motion will take place between the mass and the supporting body; and still further,that when the pressure is lessened, the ball approaches the spherical form, finally attaining that form when all pressure has been removed. Now, if we approach this subject free from preconceived notions, to observe what has taken place, we find that when mass motion begins to diminish, what we call pressure begins to increase, and when mass motion begins to increase, pressure diminishes. All we have to show for this so-called pressure is change of form. Change of form implies molecular displacement, and molecular displacement implies increased mol-cular motion ; so that, in this case, we have the simple conversion of mass motion into molecular motion. In this view, pressure is simply increased molecular motion, and is synonymous with tension. Let us now try our experiment with a liquid inclosed in a vertical pipe, with a pressure gage inserted at the bottom. Let the pipe 1'ave a funnel at the> top, into which water may flow to maintain a given head while the water is flowing through the tube, and an escape pipe to keep it down to the same head when the bottom of the tube is closed. If now the bottom be closed, the pressure gage will show an amount of pressure upon the sides of the pipe corresponding to the hight of the water column. And if the tube be composed of elastic rubber, we shall find that it expands until its resisting force is equal to the pressure upon its walls. If, now, the water be permitted to flow through a tap at the bottom, we shall find the pressure, indicated by the pressure gage, to decrease,while the elastic walls of the tube contract. We have here the same phenomena of diminished pressure,increased motion and change of form, we had in the first experiment. With gases we also find pressure diminishing in the tubes through which they are conveyed, as motion of the column increased. So after having examined the three states of matter, solid, liquid, and gaseous, we find that in all observed cases, diminution of mass motion, and increase of pressure are connected, and vice versa; while at the same time we find that all we can discover of pressure is a change of form in masses, greater or less as the so-called pressure is greater or less. Now, whether we regard this change of form as the result of an occult force styled pressure, "vis mortua," or anything else we please to call it ; or whether we reject the hypothesis of an occult force, and consent to consider pressure and tension as identical, and the representative of molecular motion, does not affect the fundamental truth that pressure, as pressure, never performs work, and that only when it partially or wholly ceases, mass motion, which is capable of being directly applied to work, supervenes. So that if it were possible for our Australian inventor to convert all the pressure which now sustains his vessel upon the surface into mass motion or direct power to perform work, the supporting power would be gone with the pressure, and though his vessel might go ahead, it would also go—to the bottom. Any portion of this pressure converted into motion, is so much subtraction from its supporting power, which is pressure. The necessity for a more perfect understanding of the fundamental laws of physics on the part of inventors cannot he too strongly urged. The ground upon which nuggets of discovery, so to speak, could be picked up at random, by any oie who had eyes, has been mostly explored. There are now nc more nuggets. Those who get gold hereafter must mine scientifically, or return with empty pockets.