181

ERRATA.

Page 25, Ex. 17, for velocity read acceleration.
Page 27, Ex. 40, for 56 lbs. read 49 lbs.
Page 111, Ex. 117 (last line), for 50° C. read - 50° C.
Page 112, Ex. 119 (last line), for volume read pressure.
Page 127, 6th line from bottom, for ms0 read mst.
Page 153, Ex. 5, for 4 yards read

4 feet.

Chap. I. (p. 247), 42, for 18 feet read 16 feet.
read 14.58 gm.

53, for 9.5 gm.

66,

205

247

Chap. II. (p. 250), 78, sp. gr. of solid=0.5; of liquid=2.

Resultant pressure=62.5 lbs.

Chap. III. (p. 252), 48, for 132°.3 read 131°.7.
Chap. IV. (p. 253), 48, for 6.25 gm. read 625 gm.
Chap. V. (p. 254), 6, for 1010 read 109.
24, 124.4 units. 47, for 2.85 read 2.87.

29, for 24,396 units read

Chap. VI. (p. 255), 26, for I ft. 1 in. read 1 ft. 27, for inverted

read erect.

Chap. VII. (p. 256), 1, 341 metres per sec.

7, 32.4 cm.

1. Units. In order to measure any physical quantity, we have first to select as our unit, or standard of reference, a quantity of the same kind as that to be measured. The ratio between the quantity and the selected unit is called the numerical value or measure of

the quantity. Suppose that we have to measure a definite length 1, and that we adopt as our unit a length L: the numerical value (n) of the length to be measured will be

n = L'

where n may be any number, whole or fractional.

2. Fundamental Units. All physical quantities can be expressed in terms of three fundamental units, the choice of which depends upon the ease and certainty with which the standard quantities so selected can be compared with other quantities of the same kind. We might choose as our fundamental quantities a definite length, a definite force, and a definite interval of time: other units, such as those of mass and work, could be deduced from these. But on account of the difficulty of devising a permanent standard of force, the value of which would not change from place to place, such a

The fundamental units usually adopted are thos length, mass, and time; these three elements can measured with great accuracy, and standards of le and mass can easily be copied, and compared with original standards.

THE C.G.S. SYSTEM OF UNITS.

A committee of the British Association has rec mended the adoption of the centimetre, the gram and the second as the three fundamental units. Of units derived from these are distinguished by the let "C.G.S." prefixed, these being the initial letters of three fundamental units.

Velocity. The C.G.S. unit of velocity is the velo of a point which moves over one centimetre i second.

Acceleration. The C.G.S. unit of acceleration that of a point whose velocity increases by unit per second. The numerical value of acceleration due to gravity (g) is 978.10 at equator, 980.94 at Paris, 981.17 at Greenw and 983.11 at the pole.

Force. The C.G.S. unit of force is that force wh
acting upon a mass of one gramme for a secc
generates in it a velocity of one centimetre
second.

Special names are given to some of these un
thus the C.G.S. unit of force is called the d
Assuming the value of g to be 981 (as we s
do throughout), we see that a dyne is
weight of a gramme.