solar radiation as the basis of all terrestrial physics, and upon radiodynamics, or the science which refers all physical activity to centres of energy, as the universal physical science. Gravitation, cohesion and chemical affinity are directly concerned only with centripetal phases of force; inertia, in orbital and explosive motions, introduces a kind of centrifugal action; heat, properly speaking, seems to be wholly centrifugal, for the approach of particles when heat is radiated can hardly be attributed to thermo-dynamic action; electricity and magnetism, as positive and negative, boreal and austral, are both centripetal and centrifugal; light, according to the undulatory hypothesis, also represents both phases of activity, in the alternate contractions and expansions of wave propagation, as well as in the phenomena of radiation, refraction, reflection and coloration.

Electricity and light have been connected, and to some extent identified, by means of investigations which were begun by Weber and Kohlrausch, in Germany, and continued by Thomson, Maxwell, Ayrton and Perry, in England. As a result of those investigations, it has been found that electro-magnetism is related to electro-statics, somewhat as momentum to mass, the electro-magnetic unit being equivalent to the electro-static unit multiplied by the velocity of light.

Maxwell, accordingly, regarded light as an electro-magnetic phenomenon. It seems to me more logical to regard electro-magnetism as a luminous or radial phenomenon, for the following reasons:

1. Because the velocity of light is only one factor of electro-magnetism, but it is the important factor which constitutes it a force.

2. Because we have no evidence of electro-magnetic action in space, while we have much evidence of the action of light.

3. Because the eminent practical observers, who have studied the phenomena of terrestrial magnetism most carefully, have concluded that there is no specific magnetism in the sun and moon to influence the terrestrial magnetism through induction.

4. Because the mass-factor, which constitutes an important though subordinate element in all thermal, chemical, electrical and magnetic phenomena, is mainly, at least so far as it appears most obviously in those phenomena, a terrestrial factor.

5. Because it is better to designate the solar radiations by a name which will be universally recognized as appropriate, than by a name which has been generally applied only to local phenomena.

A still stronger and perhaps conclusive reason for regarding photodynamics as a special and principal department of radio-dynamics, is the fact that the velocity of light, as I propose to show you, is an important factor of gravitating, as well as of electro-magnetic action. In studying the phenomena of gravitation, there is no necessity for introducing any other elements than those of simple vis viva, mass and the square of the velocity. If the limit of efficient velocity can be shown to be the velocity of light in both departments, the law of parsimony would exclude the electro-static unit, unless it can be shown that it is a necessary element of mass. This has never yet been done. If the necessity should be demonstrated hereafter, it is more likely that it will be found to depend upon some modification of the fundamental velocity of light than upon any independent activity which can be regarded as purely electrical.

The chief postulate of photo-dynamics may be stated as follows : All physical phenomena are due to an Omnipresent Power, acting in ways which may be represented by harmonic or cyclical undulations in an elastic medium.

The Omnipresent Power is scientifically required by the law of harmony; the harmonic or cyclical undulations, by the law of permanence or stability; the representative elastic medium, by the law of equal and opposite action and reaction. All questions as to the reality or nature of the supposed medium are of minor importance. Although my investigations have strengthened my own belief in the reality of an all-pervading æther, we are only required to recognize the existence of phenomena which involve such actions, and can be explained by such laws, as have been deduced from the motions of the atmosphere and other elastic fluids.

The following well-known laws have an important bearing upon photo-dynamics:

1. Cyclical activities may often be accurately represented by formulas which introduce mean or average velocities and mean vis viva. This is the foundation of Maxwell's theory of the equality of mean vis viva in the molecular movements of different gases at equal temperatures, and of Pfaundler's discovery that in estimating the heat of dissociation, the mean should be taken between the temperatures of incipient and of complete dissociation.

2. The projectile force, which produces flight or cyclical motion against any central acceleration or retardation, is equivalent to the

mean acceleration or retardation multiplied by one-half the time of flight or cyclical motion.

3. The velocities of wave motion in elastic fluids, and of cosmical and molecular orbital motion, can all be expressed by a common formula.

4. Every periodic vibrating or orbital motion can be regarded as the sum of a certain number of pendulum vibrations.

5. Mean vis viva may be represented by the vis viva of centres of oscillation.

6. The distance of the centre of oscillation from the centre of relative stability is at two-thirds of the length of a linear pendulum, or at the square root of four-tenths of radius in a rotating sphere.

7. The acceleration of any force, which is uniformly diffused from or towards a given centre, varies inversely as the square of the distance from the centre.

8. Times of revolution, under the action of such forces, vary as the three halves power of the distance; distances vary as the two-thirds power of the time.

9. Centres of inertia, or nodes, in a vibrating elastic medium, tend to produce harmonic nodes.

10. The force of planetary projection should be referred to perihelion; the force of incipient subsidence, to aphelion.

11. The mutual inter-actions of cosmical, molecular or atomic bodies are proportioned to the respective masses; actions which are considered with reference to a single active centre vary directly as the mass and inversely as the square of the distance.

12. In elastic atmospheres the densities decrease in geometrical progression, as the height above the surface increases in arithmetical progression.

13. Living force, or vis viva, is proportional to the product of mass by the square of the velocity.

14. The distance of projection against uniform resistance is proportioned to the living force.

15. In synchronous orbits, the mean velocity of rectlinear oscillation is to the velocity of circular orbital oscillation as twice the diameter is to the circumference.

16. In a condensing nebula, the velocity of circular orbital revolution is acquired by subsidence, from a state of rest, through one-half of radius.

The following additional propositions may be readily deduced from the foregoing.

17. The acceleration or retardation of a centripetal force varies as the fourth power of the velocity of orbital revolution.

18. In cyclical motions, the resultant of all internal forces must be in equilibrium with the resultant of all external forces, at the expiration of each half cycle.

19. The modulus of cyclical motion is equal to the product of acceleration by the square of the time of a half cycle.

20. The sum of all external forces may, therefore, be represented (2) by a velocity which is equivalent to the mean or resultant internal force acting for one-half of the cyclical time.

21. At the extremity of a linear pendulum, the influence of a central force on the centre of oscillation is nine times as great as on the centre of suspension.

22. The limiting vis viva of wave propagation is five-ninths of the mean vis viva of the oscillating particles.

23. In condensing nebula, rupturing forces which are due to central subsidence may be represented by fractions in which the denominator is one greater than the numerator,

24. In synchronous rotation and revolution, the nucleal radius varies as the three-fourths power of the limiting atmospheric radius. 25. The variation in mean vis viva of gaseous volume is to the variation in vis viva of uniform velocity as 1 is to 1.4232.

26. The mean thermal and mechanical influences of the sun must be in equilibrium.

27. The collisions of particles, in subsiding towards a centre of force, tend to form belts at the centre of linear oscillation.

28. The limiting velocity between tendencies to aggregation and tendencies to dissociation is to the velocity in a circular orbit as the ratio of the circumference of a circle to its diameter is to the square root of two.

29. In explosive, as well as in cyclical motions, equilibrium must be established between internal and external forces.

30. Apsidal and mean planetary positions must also be controlled by like tendencies to equilibrium.

31. Undulations in an elastic medium maintain the primitive velocity which is due to their place of origination.

32. When two or more cyclical motions are combined, they must all be modified by the tendency to conservation of areas.

• 33. In expanding or condensing nebulæ, the conservation of areas maintains a constant value for the modulus of rotation.

34. Instantaneous action between different masses or particles by mere material intervention is impossible.

35. In synchronous motions about different centres, the mean distances from the centres of motion vary as the cube root of the masses or other controlling forces.

36. Constant velocities, in a homogeneous elastic medium, represent constant living forces.

In applying these general principles, we must expect to meet with perturbations, arising from the adjustment of opposing tendencies. If the problem of three bodies is so difficult, in astronomy, as to defy all efforts at satisfactory solution, the attempt to grapple with all the intricacies of elastic interaction may also defy the ordinary methods. of mathematical analysis. And yet, by paying proper regard to mean values, it is possible, through very brief and simple processes, to get approximate determinations of important astronomical and physical constants, in which the error is less than in the ordinary approximations which require long, tedious and intricate calculations.

Lockyer's late spectroscopic researches have awakened a new interest in the old theory, that all chemical elements are merely different forms of condensed æther, and that the æther itself is only a universal atmosphere. Taking this theory as a provisional hypothesis, there can be little question that hydrogen is the element which resembles the æther most closely, and which may, therefore, be regarded either as the first step in elementary condensation, or as the transmitter of primordial undulation. It is the lightest of all known substances; it is hyperelastic, being the only gas in which the elasticity increases faster than the condensation; it is always present in solar explosions, if the evidence of the spectroscope is trustworthy; the height to which it is thrown, and the rapidity of its diffusion, in these explosions, indicate a force and velocity which can be best explained by photo-dynamic influence; there are many reasons for believing that it is the outer envelope of the sun; and it presents many features of peculiar interest in connection with Lockyer's basic lines, which furnish simple harmonic indications of great significance.

In order to illustrate some of the properties of hydrogen I have