_Third_. An interesting solution is presented here of what is to many an astronomical puzzle. When I was younger than I am now, I was greatly troubled to understand how it could be that if the moon was always falling to the earth, as the astronomers assured us it was, it should never reach it, nor have its falling velocity accelerated. In popular treatises on astronomy, such for example as that of Professor Newcomb, this is explained by a diagram in which the tangential line is carried out as in Fig. 1, and by showing that in falling from the point A to the earth as a center, through distances increasing as the square of the time, the moon, having the tangential velocity that it has, could never get nearer to the earth than the circle in which it revolves around it. This is all very true, and very unsatisfactory. We know that this long tangential line has nothing to do with the motion of the moon, and while we are compelled to assent to the demonstration, we want something better. To my mind the better and more satisfactory explanation is found in the fact that the moon is forever commencing to fall, and is continually beginning to fall in a new direction. A revolving body, as we have seen, never gets past that point, which is entirely beyond our sight and our comprehension, of beginning to fall, before the direction of its fall is changed. So, under the attraction of the earth, the moon is forever leaving a new tangential direction of motion at the same rate, without acceleration. (_To be continued_.) * * * * * COMPRESSED AIR POWER SCHEMES. By J. STURGEON, Engineer of the Birmingham Compressed Air Power Company. In the article on "Gas, Air, and Water Power" in the _Journal_ for Dec. 8 last, you state that you await with some curiosity my reply to certain points in reference to the compressed air power schemes alluded to in that article. I now, therefore, take the liberty of submitting to you the arguments on my side of the question (which are substantially the same as those I am submitting to Mr. Hewson, the Borough Engineer of Leeds). The details and estimates for the Leeds scheme are not yet in a forward enough state to enable me to give them at present; but the whole case is sufficiently worked out for Birmingham to enable a fair deduction to be made therefrom as regards the utility of the system in other towns. In Birmingham, progress has been delayed owing to difficulties in procuring a site for the works, and other matters of detail. We have, however, recently succeeded in obtaining a suitable place, and making arrangements for railway siding, water supply, etc.; and we hope to be in a position to start early in the present year.