We are always glad to give our correspondents a hearing upon subjects which we consider likely to be beneficial to our readers at large, but we find it necessary occasionally to hold our correspondents in check. At the present moment we find our desk loaded down with articles upon purely speculative topics, involving abstract theories which have puzzled the wisest of all ages. We cannot give up our columns to the discussion of such subjects, asverylittlecanbe evolved therefrom, eitHer new or profitable to our readers. We prefer something practical, something that shall add to the general stock of useful knowledge, and aid in promoting and developing the industries of the world. Let us enumerate some of the topics contained in this heap of rejected correspondence. We have several upon the " Fluid Character of Electricity;" another upon *' The Cause of the Attraction of the Positive Pole of a Magnet for the Negative Pole;" another, which comprises as nearly as possible all the speculative inquiries of past and present ages, and which demands answers to no less than seventeen " whys," all pertaining to force considered as an abstract entity, and the origin of all existence; another upon the " Origin of all the Forces upon the Earth; " another upon the " Solidity of the Earth's Center," and so on to the bottom of the pile. Every one of these letters has been carefully read and considered. What possible good can arise from the discussion of these and cognate subjects ? We maintain that " why matter is and why matter moves " must, from the very nature of the case, remain beyond the pale of legitimate physical science, whose province it is to investigate the manner and succession in which natural events transpire, and not why things exist. The latter inquiry is either a subject for religious belief, or of speculative and transcendental philosophy, if that deserves' the name of philosophy which is founded upon mere hypothesis. We know nothing of abstract force except by inference, if inference can be called knowledge. All that we can demonstrate is that matter under certain conditions moves in a manner always the same when the conditions are the same. This relation of motion to the conditions which precede it, is what we call law, a term which, in its physical sense, means only the constant relation which exists between any particular motion and the perceptible conditions under which it takes place. So far as we can see, matter and motion are always connected. Ef this is the result of an occult force, we know nothing of that force, and consider it impossible to demonstrate its existence. If its existence be admitted, we consider it just as legitimate a subject of philosophical inquiry to ask what underlies; hat force, and so on without end. If a first cause for matter md motion is a necessity of thought, what is the use of supposing intermediate causes between the first cause and matter ?ind motion ? We must finally stop at a cause uncaused, if we substitute a thousand intermediate causes. Why not say God created mafter and put it in motion, and out of these facts, matter and motion, we have our universe ? In this view, as soon as we step beyond matter and motion, we are in the presence of the first cause, Deity himself, and beyond the realm of physics. But it may be said the speculative theories tf-hich have suggested our correspondents' inquiries pertain, it least some of them, to this realm. We consider them of 10 greater value on that account. If their tendency was to point the way to probable discovery, they might be of some ralue; but so far as we can see, they do not: their discussion; an therefore be of no benefit. We trust our readers will not permit their attention to be distracted from practical ques* tions. It is futile to seek by scientific methods for the " why " of existence, but we may find out the " how " of many things that will confer good upon ourselves and our fellow men. This is the lesson we set out to teach.