stand, with centre of gravity and magnitude; 7, mechanical paradoxa double cone appears to roll up hill; 8, horseman, balanced on two points. This set also includes a brass plumb, cord and handle, for supporting the various articles on centre of gravity. Price of set, $8.75. 47. The Brachystochrone, or line of swiftest descent; six feet in length, made of mahogany, with two ivory balls. The curved path is that of a cycloid, and when two balls are released from the top of the inclined plane and the cycloidial curve at the same time, the latter will reach the bottom first, although the path in which it rolls is the longest. Price $10. 48. Prismatic Cylinder to recompose white light. This is attached to the centrifugal machine, pp. 86, Descriptive Catalogue. Price $1.65. 49. Falling Bodies.-Apparatus for illustrating all the principles of the laws of falling bodies, with a set of apparatus attached to illustrate the theory of the pendulum. Price $30. 50. Centre of Gravity.-Two parallelopipeds of a rhomboidal form, to illustrate centre of gravity. They stand firmly on end when separate, but when placed on one another, they are on the point of falling. Price 25 cts. 51. Oblique Cylinder in two pieces to illustrate the same as preceding. Price 25 cts. 52. Screw.-Apparatus for illustrating the principle of the action of the screw; size of the screw 6 inches long, 3 inches diameter, with square bottomed thread, mounted on a spindle with handle. Price $1.25. 53. Lock.-Large wooden model of a lock, with spring and tumbler, mounted on a blackboard, 15 inches by 10 inches, with an iron key. Price $1.80. 57. Hiero's Fountain, of copper, 12 inches diameter, 40 inches high, with stop cocks, &c. Price $6.50. 58. Hiero's Fountain, of glass, by which the operation is seen. Price $8.50 to $12. 59. Archimedes' Screw Pump, with stand and cistern. Price $2.70 to $5.40. 60. Barker's Mill, the moving cistern with four sprays; mounted on a frame. Price $1.80. 61. Centrifugal Pump, Glass model of the. Price $7. 62. Glass Syphon of the common form. Price 15 cts. 63. Tate's Mercurial Hydraulic Pump, consisting of a bent glass tube with a metal cistern and a wooden plunger. This pump works with about lb. of mercury put into the end of the tube. It requires no valve. It must be used with a tall glass cylinder kept nearly full of water. In pumping, the downward motion must be slow and the upward quick, Price $1.25. STEAM. 64. Wightman's Sectional Model of Watt's Low-Pressure Steam Engine and Boiler, with Furnace. Beam 16 inches.......... The parts in this model are truly sectional—the appearance of one side being an exterior view, while the reverse shows the interior, with the piston and valves in motion, as in the real machine. 65. Miniature Steam Engine, High-Pressure, of Brass $38 00 ... 13 00 This model is put into motion by a small spirit lamp, and is 66. Revolving Steam Jet, of Brass, complete in itself; illustrates .... 1970 0 09 PNEUMATICS. 68. Air Pump: basement of Mahogany; two barrels ; plate 8 inches, barrel 7 by 2 inches; works by a double lever, with bell glass... 69. Double Acting Condenser and Exhauster; barrel 7 inches by 1; the change is effected by simply turning the base half round... 50 00 870 70. Condenser; barrel 7 inches by 1; by reversing the piston and Valve, it acts as an Exhauster... .... 85 40 71. Tate's Double Action Air Pump; length of barrel 16 inches, bore 14, stroke 8 inches; will freeze water over sulphuric acid under a receiver of 300 cubic inches in 150 strokes; it is mounted on a massive brass clamp with a transfer plate, &c., to convert the apparatus into a condensing pump.. ... 18 00 72. Swelled Bell Glasses; 4 sizes; one gallon, $1.50; two gallons $2.40; four gallons $4.20; eight gallons .... 73. Swelled open top Bell Glasses, with glass covers; capacities, ... 8 50 4 80 540 75. Hand Glass to show pressure of air for mercury tunnel, &c. ... 76. Stand, Lever and Fulcrum, used with hemispherical cup for weighing a column of air...... 0.90 5 40 77. Freezing Apparatus, bell glass, pan for acid, improved silvered water cup and stand..... 4 80 79. Double revolving Jet, revolves in opposite directions 2.20 80. Bell for vacuo, with stand; the bell is entirely insulated. $1.40 and 275 81. Brass Plate and Wood Cylinder; illustrating the porosity of wood, pressure of air, &c...... 1 10 82. Lock for striking flint and steel in vacuum. 2 20 83. Barometer in vacuo; bell glass, 33 inches high, tube, cup and сар... 3 60 84. Artificial Fountain, or fountain in vacuo, with stopcock and jet, stand, &c., 20 inches.... 4 50 $4.60 85. Guinea and feather apparatus, with tall conical receiver, sliding rod plate, drop tables, &c; 3 feet, $8.40; 4 feet.... 12 00 86. Guinea and feather apparatus; two falls, ground brass plate and stuffing box; extra hook for use with the Bell, &c.......... 87. Guinea and feather tube; improved, capped at each end, with stopcock and stand, and made heavy and strong for showing the resistance of condensed air [also fitted for Aurora Tube for Electricity]; 2 feet, $4.80; 8 feet 750 88. Mercury Tunnel for showing the porosity of wood, pressure of air, &c. 085 UPWARD PRESSURE APPARATUS. 89, Upward Pressure Apparatus; glass bell; brass cap, with India rubber bag, to which weights are attached; tripod stand and strap... 540 1 80 90. Bursting Squares; for expansion or pressure, per doz....... 91. Wire Guard for Bursting Squares.... 0 60 92. Water Hammer; hermetically sealed, showing that the collision of water in a vacuum produces a sharp noise like solid bodies.. 93. Sliding Rod; with packing screw, regulating screws, hook and pincers, &c... 1 20 1 60 94. Sliding Rod, with Packing Screw. 1 20 95. Sheet Rubber Bag, with cap and hook, $1.35; with stop cock 2 20 ......... 10 80 1 63 |