Suspend a bag of gunpowder containing 50 or 60 lbs., near the middle of the gate, upon a nail or gimlet, having a small piece of port-fire inserted at the bottom, and well secured with twine. 41. What is the length of a pendulum to vibrate seconds, half and quarter seconds respectively? Seconds, 39.1 inches; half-seconds, 9.8 inches; and quarter-seconds, 2.45 inches. [Sidenote: 159]] 42. Give a formula for determining the length of the seconds pendulum in any latitude. 1 _l_ = ————————— [32.1803 feet - 0.0821 cos. 2 lat.] 9.8696044 43. How are the times of a single oscillation of two pendulums to each other? As the square roots of their lengths. 44. Repeat the table of measures. 10 tenths, 1 inch. 4 inches, 1 hand. 12 inches, 1 foot. 28 inches, 1 pace. 3 feet, 1 yard. 2 yards, 1 fathom. 220 yards, 1 furlong. 1760 yards, 1 mile. 45. Repeat the table of avoirdupois weight. 47.34735 grains, 1 dram. 16 drams, 1 ounce. 16 ounces, 1 pound. 28 pounds, 1 quarter. 4 qrs. or 112 lbs. 1 cwt. 20 cwt. 1 ton. In some of our States the ton is estimated at 2000 lbs. 46. What is the force of gravity? It is that force of attraction exerted by the earth upon all particles of matter which tends to urge them towards its centre. 47. What is the specific gravity of a body? The ratio of the weight of a body to that of an equal volume of some other body assumed as a standard, usually pure distilled water at a certain temperature. 48. What is the law of descent of falling bodies? The spaces fallen through from the commencement of the descent are proportional to the squares of the times elapsed. [Sidenote: 160]] 49. What compositions are made use of for preserving iron cannon? 1. Black lead, pulverized, 12 Red lead, 12 Litharge, 5 Lampblack, 5 Linseed Oil, 66 Boil it gently about twenty minutes, during which time it must be constantly stirred. 2. Umber, ground, 3.75 Gum Shellac, pulverized, 3.75 Ivory black, 3.75 Litharge, 3.75 Linseed Oil, 78 Spirits of turpentine. 7.25 The oil must be first boiled half an hour; the mixture is then boiled 24 hours, poured off from the sediment, and put in jugs, corked. 3. Coal tar (of good quality), 2 gals. Spirits of turpentine, 1 pint.