The Bioscope (Jan-Mar 1913)

THE Bioscope, JANUARY 16, 1913. is set in an eccentric bearing, by the turning of which the cross can be moved a little nearer to the cam. The eccentric bearing is. of course, controlled by screws, and by careful adjustment of these the cross can be brought to the correct position. In other machines the cross bearings are set in a bracket, which, by means of adjusting screws, can be swung nearer to or further from .the cam. When correctly adjusted the actuating cam should turn frecly against one of the curved faces of the cross. When carrying out an adjustment of this nature it should be done carefully, and the four sides of the cross tested before running the machine, not forgetung to lubricate the parts. If the trouble be not due to the Maltese Cross, put in new gate’ springs, which in all probability through the heat of the gate have lost their tension. Attention to these two points will remedy an unsteady picture, but, needless to say, the gate surfaces must be kept clean; and also, an important thing, no dirt must be allowed to collect around the teeth of the intermittent sprocket wheel, or, however nicely the Maltese Cross be adjusted, the results of projection would not be of the best. kK ok * PETROL writes: “I have lately had trouble with my petrol-driven generating plant. I find it most difficult to start up the engine, taking, in fact, on many occasions half-an-hour before it will fire. I also find that when running on full load, with the throttle wide open, the engine thumps very considerably. The cngine has been dismantled, and I am told that all the bearings are in good order.” The difficulty you have in starting is one experienced with most petrol engines in cold weather, when less air is required for the mixture than in warm weather. With many carburetters it is impossible to cut off sufficient air to form an explosive mixture for starting, and unless an additional valve be fitced for reducing the air passing through the induction pipe, some temporary arrangement 1s necessary to effect a speedy start. A common practice is to put a rag over the end ot the inlet pipe, when little difficulty will be experienced in getting an initial explosion. Of course, after the first few revolutions, the tag can be removed. Another and better plan is to make a saw cut in the induction pipe, in which may be fitted a tin slide. This will generally enable the engine to be started up in the coldest weather with the first swing of the crank. Regarding the knock, if the bearings, including the top and bottom ends of the connecting rod, be all well-fitting, your trouble is due to one of two causes—either the spark is too far advanced for the speed of the engine, or pre-ignition takes place. As no doubt you have seen to it that the electric ignition conditions are the same as Google hitherto, it will in all probability be the lattermentioned that is causing your trouble. Most likely there is a thick carbon deposit inside the cylinder head and on the piston crown, which becomes so heated that the gases ignite prior to the piston reaching the end of the compression stroke. The remedy is to take the cylinder or cylinders down, and to well scrape off any carbon deposit there may be. It sometimes happens that pre-ignition takes place even when the cylinder is absolutely clean, but this is due to some small projecting piece, such as a burr in the casting, becoming red-hot, and so firing the gases. We think, however, you will find _ that your trouble is due to formation of a car bonous deposit. * * ‘““S.K.” wishes to know what proportion should exist between the focal lengths of cinematograph projection and lantern slide lenses so as to fill exactly the same size screen. The first essential for a lantern slide picture ‘to fill exact/); the same space as that occupied by the cinematograph picture is that the proportion of height to width of lantern slide opening should be the same as height to width of the cinematograph mask. This is very rarely the case. In fact, whereas the cinematograph mask opening is an oblong laid horizontally, the majority of lantern slides, such as the “ King” slides, shown in picture theatres, are vertical oblongs. However, bearing in mind that it is almost an impossibility to get every lantern slide masked to the same proportion, it is best to consider an ordinary bound lantern slide having practically a 3 in. square opening. The _ proportion of this to the width of projector gate width is three to one, and to projector gate height four to one. So, to make such a slide project upon the screen, the same width of picture as the cinema one, a lens of three times the focal length to that used for the film projector must be employed, whilst to make the same lantern slide fall within the eight of the cinema screen, a lens of four times the focal length is necessary. Tor example, if a 4 in. projection lens be employed, and it is desired to project a lantern slide, not masked down, of the same width as the screen, a 12 in. lantern lens must be used, whereas to obtain a lantern picture of the same height as the screen, a 16 in. lens is necessary. In the first-named case the top and bottom of the lantern slide would be projected on the border of the screen, whilst in the latter case the sides of the screen would be without any picture. However. it must be remembered that lantern slides are variously masked, so that, although the four to one proportion of lenses will always insure the lantern slide picture falling inside the screcn, the three to one proportion will, in the great majority of cases, suffice.