Identifier: workofprac00algegroundrich (find matches)
Title: The groundwork of practical naval gunnery; a study of the principles and practice of exterior ballistics, as applied to naval gunnery, and of the computation and use of ballistic and range tables
Year: 1917 (1910s)
Authors: Alger, Philip Rounseville, 1859-1912 United States. Naval Academy, Annapolis. Dept. of Ordnance and Gunnery Ingalls, James M. (James Monroe), 1837-1927
Subjects: Ballistics
Publisher: Annapolis, Md., The United States Naval Institute
Contributing Library: University of California Libraries
Digitizing Sponsor: MSN

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ur usual naval gun mounts; and, as the value ofD to be allowed for is seldom as closely known as that, it is evident that in direct fire,under all ordinary circumstances, we may use (196) * D and X must be in the same units, either both feet or both yards, d will then comein the same units as I, usually inches. / 150 EXTERIOR BALLISTICS Permanentangle. 225. It was common practice with naval ^ns and bar sights to correct for thegreater part of the drift automatically by inclining the bar sight in a plane per-pendicular to the axis of the bore of the gun, so as to make with the vertical anangle i called the permanent angle. Referring to Figure 17, we see that the threepoints D, C and C are in the same plane, which is perpendicular to the line CB, theangles CCD and BCD being right angles; the points B, C and C are all three in thesame plane, which is at right angles to DCC; B, C and D are in the same plane,which is at right angles to BCC; and, similarly, for P, P and Q. Then we have that
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CB = l, being the natural line of sight, and CD = h is the sight bar height for theangle of departure , and is now given by . h = lUTicf>seci ^ (197) Then if PF be the drift in yards, at the range R, from the similar triangles we have —^ = ri ; but DC = h sin i = Z tan <^ tan i, and CB = I sec . ThereforeC B R tanj> tan t _^.^ d, tan i= -^ , whence we havesec 4* ^ tan i — D R sin ) ^ (198) 226. If D were proportional to X sin , which it is not far from being, settingthe sight bar at the permanent angle i given by (198) would exactly compensate fordrift at all ranges. Actually, however, D increases a little more rapidly in propor-tion than Z sin , and so the sight should be more inclined for long than for shortranges. In practice, when bar sights are used, it is customary to compute the value * D and R must be in the same units. In the above equation they are both expressedIn yards. VARIATION OF THE TRAJECTORY FROM A PLANE CURV

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