the telescope, move a lighted lamp or match about until a glimpse of it is caught reflected in the mirror. The position of the lamp at that moment shews you where the scale should be.

(If the first method be adopted, it is easier to see the scale by going close up to the mirror until it comes into view, and then moving backwards to the telescope, still keeping it in sight.)

Suppose now the scale is seen reflected from the mirror; the central division of the scale will probably not coincide with the cross-wire.

For many purposes this is unimportant. If, however, we wish to bring the two together we must notice what point on the scale will come opposite the cross-wire when the mirror is at rest,' and then turn the torsion head, which carries the wire in the right direction until the central division is ught into view.

t may be impossible to make the adjustment in this. iner; in that case we must move the telescope and scale. *", it o be the central division of the scale and P the which coincides with the cross-wire, the necessary ent will be made if we move the telescope and scale half the distance o P, still keeping the former pointed. mirior.

is sometimes necessary to set the scale at right angles or MG. For this purpose measure with a string or A the distances of EM and F M, and turn the scale round al axis until these two are equal. Then since OE=OF e=M F, it is clear that o M and EF are at right angles, 'uired adjustments are complete.

jus

the

em

1e the Time of a Complete Vibration.

the bar slightly from its position of rest, and let it

hen the position on the scale of the turning-points' of the can be read through the telescope, the position of equilibrium can ermined in exactly the same manner as in the case of the balance 312).

The scale will appear to cross the field of view of the telescope.

Note with a watch or chronometer the instant at which the middle point of the scale passes the cross-wire of the telescope, marking also the direction in which the scale appears to be moving. Let us suppose it is from left to right. It is of course impossible to see at the same time the cross-wire and scale and also the face of the chronometer; but the observation may be effected either as described in § 11 or as follows.

Let us suppose the chronometer ticks half-seconds.

Listen carefully for the sound of the tick next after the transit of the central division of the scale, and count six in time with the ticks, moving at the same time the eye from the telescope to the clock-face. Suppose that at the sixth tick the chronometer registers 10 h. 25 min. 31°5 sec., then the instant of transit was 3 sec. earlier, or 10 h. 25 min. 28.5 sec. Raise the eye quickly back to the telescope and watch for the next transit from left to right.

Again count six ticks, moving the eye to the chronometer, and let the time be 10 h. 26 min. 22 sec.

The time of the second transit is then 10 h. 26 min. 19 sec., and the time of a complete vibration is 50`5 sec. But either observation may be wrong by 5 sec., so that this result is only accurate to within 1 sec.

To obtain a more accurate result proceed exactly as in § 20.

It may happen that the time of vibration is so short that we have not time to perform all the necessary operations -namely, to move the eye from the telescope, look at the chronometer, note the result, and be ready for another transit before that transit occurs. In such a case we must observe every second or third transit instead of each one.

Again, we may find that 6 ticks do not give time to move the eye from the telescope to the chronometer-face. If this be so, we must take 8 or 10. Practice, however, soon renders the work more rapid.

Of course, if we always count the same number of ticks there is no need to subtract the 3 sec. from the chronometer reading; we are concerned only with the differences between the times of transit, and the 3 sec. affects all alike.

We may thus observe 1, the time of vibration of the needle when the empty tubes are nearest the ends, the loaded tubes being in the middle; and in the same manner we may observe t2, the time of vibration of the needle when the positions of the heavy and light tubes have been interchanged. Let the observed value of t, be 17.496 sec., and that of 2, 25.263 sec.

To find the Value of k, the Increase in the Moment of Inertia.

We know that the moment of inertia of a body about any axis is equal to its moment of inertia about a parallel axis through its centre of gravity, together with the moment of inertia of the whole mass collected at its centre of gravity about the given axis (p. 44).

Thus, let m be the mass of a body whose moment of inertia about a certain axis is I; let a be the distance of the centre of gravity from that axis, and I the moment of inertia about a parallel axis through the centre of gravity. Then I=I+ma2.

Moreover, the moment of inertia of a body is the sum of the moments of inertia of its parts (p. 44).

Now, let m, be the mass of each of the heavy tubes, and a the distance of the centre of each of them from the axis round which the whole is twisting when in the first position. Let I, be the moment of inertia of each of the heavy tubes about a parallel axis through its centre. Let m2, I have the same meaning for the empty tubes, and let b be the distance of the centre of each of these from the axis of rotation. Let I be the moment of inertia of the empty case. Then

In the second position, a is the distance from the axis of rotation of the centre of each of the masses m2, b of that of the masses my.

To find the moment of inertia of the whole, therefore, we require simply to interchange a and b in equation (1), and this moment of inertia is K+k. Thus,

K+k=I+21, +212 +2m ̧b2 + 2 ma2.

from (1) and (2) k=2(b2 — a2) (m ̧ — m2).

(2).

Thus, we do not need to know I, I, or I, to find k. Now the length of each of the tubes is one-fourth of that of the whole bar A B. Calling this c, we have

To find m, and m2, we require merely to determine by weighing the number of grammes which each contains. Our formula for T (p. 191) becomes

and it only remains to measure 7. This can be done by means of the beam compass or a measuring tape.

We must, of course, measure from the point at which the upper end of the wire is attached, to the point at which it is clipped by the screw D.

The wire it will be found fits into a socket at the top of the apparatus CD. Be careful when fixing it initially to push it as far as possible into the socket; its position can then be recovered at any time.

Unloose the screw D and draw the wire from above, up through the tube which supports it, and measure its length in the ordinary manner.

T

The value of thus obtained gives the modulus of torsion for the particular specimen of wire. If the modulus of torsion for the material is required, we must make use of the addi

tional law of torsional elasticity that the torsional couple in wires of the same material, differing only in area of section, is proportional to the fourth power of the radius of the wire. To find the value of the modulus of torsion of the material, the value of must be divided by where r is the radius in centimetres (p. 186).

Experiment.-Determine the modulus of torsion of the given

MECHANICS OF LIQUIDS AND GASES.

Measurement of Fluid Pressure.

THE pressure at any point of a fluid is theoretically measured by the force exerted by the fluid upon a unit area including the point. The unit area must be so small that the pressure may be regarded as the same at every point of it, or, in other words, we must find the limiting value of the fraction obtained by dividing the force on an area enclosing the point by the numerical measure of the area, when the latter is made indefinitely small.

This theoretical method of measuring a pressure is not as a rule carried out in practice. On this system of measurement, however, it can be shewn that the pressure at any point of a fluid at rest under the action of gravity is uniform over any horizontal plane, and equal to the weight of a column of the fluid whose section is of unit area, and whose length is equal to the vertical height of the free surface of the heavy fluid above the point at which the pressure is required. The pressure is therefore numerically equal to the