EXPLANATION OF SCIENTIFIC TERMS. 



13 



can fix a point which that effect or quan- 

 tity must certainly exceed, and another 

 to which it cannot possibly arrive. These 

 points are the limits of the problem. We 

 cannot, for instance, predict the exact 

 height at which the mercury will stand 

 (at the level of the sea) in a barometer, 

 on any future day ; but we may assert, 

 from past experience, that it will be 

 somewhere between twenty-eight and 

 thirty-one inches. Again, we cannot 

 determine exactly the length of the cir- 

 cumference of a circle ; but we are certain 

 that it is greater than that of any in- 

 scribed polygon, and Jess than that of any 

 circumscribed one, however numerous 

 their sides may be. 



LIQUIDITY, the state of being liquid. 

 LIQUIDS. The medium between the solid 

 and the gaseous states is that of liquidity. 

 Liquids are fluids whose elasticity is in- 

 active, and the cohesion of whose par- 

 ticles is less towards each other than 

 their individual gravities ; so that they 

 separate by their own weight, and may 

 thus be divided drop by drop. It is hence 

 that the slightest pressure on the surface 

 of a liquid, even that of the thinnest 

 stratum of its own body, presses the 

 lower portion of the fluid equally in all 

 directions, sideways as well as down- 

 wards : and even upwards, into any 

 vessel to which it may have access, if 

 there is no other way of escape. 

 MACHINE (Latin machina, a frame or 

 contrivance) . Any complication of arti- 

 ficial bodies acting upon one another by 

 contact, through the medium and motion 

 of which any effect is produced, is a 

 machine. The initial force which puts 

 the machine in motion is called the First 

 or Prime mover. The point at which that 

 force is applied is the Acting point ; and 

 that in which the effect is produced is 

 the Working point : the machine being 

 the medium through which the power is 

 transferred, and by which it is modified 

 so as to answer the intended purpose. 

 When a simple body is the medium be- 

 tween the acting and the working points, 

 it is an Instrument. 

 ^MASS (of matter). See Volume. 

 MAXIMUM. In a variable quantity or 

 effect, that quantity or effect which is 

 the greatest possible, under the circum- 

 stances in which it is placed, is termed a 

 maximum. Thus, in respect to the sails 

 of a windmill, they may be placed at any 

 angle ; but there is one angular direction 

 on which the wind will have more power 

 than on any other, and this is, therefore, 

 termed a maximum. There are other 

 cases in which we seek for a Minimum, 

 that is, the least possible. 

 MECHANICS is that science which in- 

 vestigates the nature, laws, and effects of 

 motion and moving powers. 

 MECHANICAL POWERS are the simple 



instruments or elements of which every 

 machine, however complicated, must be 

 constructed : they are the Lever, the 

 Wheel and Axle, the Pulley, the Inclined 

 Plane, the Wedge, and the Screw. 

 MELTING POINT. That point of the 

 thermometer which indicates the heat at 

 which any particular solid becomes fluid, 

 is termed the melting point of that solid. 

 MENISCUS (Greek mene, the moon), a 

 lens which is concave on one side, and 

 convex on the other ; and so called be- 

 cause it resembles the appearance of the 

 new moon. 



MINIMUM. See Maximum. 

 MIRROR (French mirer, to look at), any 

 surface from which light is reflected, so 

 as to exhibit the images of objects placed 

 before it. It is sometimes (especially 

 when formed of polished metal) termed 

 a Speculum, the Latin term for a looking- 

 glass- 



PLANE, has a plane surface, 



such as the common looking-glass. 

 CONCAVE, has a hollow sur- 

 face, which collects the rays and reflects 

 them to a focus, in front of the mirror, 

 thereby enlarging the image of the object. 



. CONVEX, disperses the rays, 



and, in consequence, diminishes the 

 image of the object. These concave and 

 convex surfaces are formed of different 

 curves, according to the purposes in- 

 tended. See Burning-Glass. 

 MOCHO STONE. See Agate. 

 MOMENTUM, or MOMENT, is the im- 

 petus, or force of a moving body. The 

 comparative momenta of bodies are in a 

 compound ratio of their quantity of mat- 

 ter and their velocity : that is, they are 

 in proportion to the products of the mat- 

 ter and velocity, when expressed in num. 

 bers. Thus a ball of four pounds weight, 

 moving at the rate of eighteen feet in a 

 second, would have double the momen- 

 tum, that is, it would strike against an 

 object with twice the force that a ball 

 of three pounds weight, moving at the 

 rate of twelve feet per second, would do ; 

 because the first product (4 multiplied by 

 18) is double that of 3 multiplied by 12. 

 Momentum is the force of percussion. 

 See Percussion. 



MOONSTONE. See Feldspar. 

 MOTION is the passing of a body, or any 

 parts of a body, from one place to an- 

 other : we say parts of a body, because 

 in the cases of a globe turning on its axis, 

 or a wheel revolving on a pivot, the parts 

 of the body change their situation, while 

 the bodies themselves are stationary. 

 MOVING POWER. See Power. 

 NON-ELASTIC FLUIDS. See Gas and 



Liquids. 



NONIUS. See Vernier's Scale. 

 OBLATE AND OBLONG SPHEROIDS, 



See Spheroid. 

 OBTUSE ANGLE. See Angle. 





