from time to time to heights of tens or hundreds of thousands of miles. The manifestation of such tremendous energy would produce fear rather than the calm assurance which at present exists as to the constancy of the Sun's condition.

Because the total amount of heat received annually from the Sun has not varied appreciably for the past two thousand years or so, we are accustomed to assume that this uniformity will be maintained. But as almost nothing is definitely known as to the source of the Sun's heat, this belief in a continuity of the present conditions is purely a matter of faith that the future will be as the past. Atomic, or other changes may occur at any time which would raise or lower the Sun's temperature sufficiently to have a profound influence upon the Earth. Any large variation of this kind would completely change the face of the world; and a decrease of ten per cent. in the amount of solar heat received would convert the temperate regions into frozen wastes.

Observations made in recent years have shown that at times the Sun does radiate more heat than at others, the amount fluctuating by about ten per cent. during irregular periods of from five to ten days' duration. These changes are possibly due to local differences of temperature on the Sun, but they may become general and progressive in one direction or another. In fact, though theoretical considerations suggest that the Sun's heat will remain practically constant for millions of years, no one can state decisively that it will do so for a hundred years.

Astronomy thus gives no support to the mind that uses the knowledge existing at any epoch as if it were a complete inventory of the universe. We can reason about things known to exist, but we must remember

that beyond the phenomena which have so far been brought within the range of our senses there may be many others awaiting to be revealed by more potent or refined means of observation, or new methods of inquiry. During the past few years, for example, the conviction has been forced upon astronomers that there are probably more dark stars than bright; and that completely blank regions here and there in the sky represent immense clouds of non-luminous cosmic dust which blot out the light of stars behind them.

Masses of material have been revealed by photography which are beyond the visual reach of the largest telescopes; many bright stars have been found to possess acolytes ponderous enough to influence their movements but without intrinsic luminosity; and the millions of meteoritic particles which enter the earth's atmosphere daily, producing the appearance of shooting stars or meteors, show that space should not be regarded as a void, but rather as a plenum of dead matter.

It is easy to understand that as the power of seeing is enlarged by the improvement of instruments or the development of methods of inquiry, new spheres are brought within the range of human knowledge. Our eyes have in truth been opened by the lenses of giant telescopes, and we have come to know that greatness is not to be measured by visibility. For not only are the stars revealed by telescopes vastly more numerous than those bright enough to be seen by the naked eye, but they also contribute more to the total quantity of starlight received by the earth than the brilliant objects which make a view of the midnight sky an impressive sight. As it is estimated that three-quarters of the total starlight that reaches this world of ours is from stars beyond the grasp of unaided vision, it is not difficult to

understand that what we see is only a limited view of the universe.

The astronomical evidence for the existence of invisible bodies in the universe is not a matter of assertion, but of interpretation of results based upon scientific inquiries. Effects are measured and are referred to physical causes which account for them in every detail, though the exact mechanism of the action may not be understood. Galileo discovered the laws of motion of falling bodies, and Newton showed that the law of gravitation is sufficient to prescribe the movements of the moon or a planet as precisely as it does for the fall of a pebble to the earth, yet the nature of gravitational attraction remains a mystery still.

From such a familiar case of relationship between fact and inference as is afforded by a falling stone, it is easy to pass to larger effects of gravitational action. For those who have little faith in intangible evidence, the discovery of the planet Neptune may be put in as a plea for confidence in conclusions based upon it. The observation of this planet in 1846 close to the position which mathematics based on the law of gravitation had assigned to it, afforded a striking instance of the ability of the law to respond to any demands which could. be made upon it in the solar system.

The planet Neptune happened to be an object bright enough to be seen with telescopic aid, though it is quite invisible to the naked eye. If no telescope had existed when this new member of our system was discovered, the actual test of the validity of the mathematical results could not have been made the processes of the intricate mathematical argument would have touched the planet and followed its movements, but, in the absence of ocular demonstration, the conclusion that a massive

object existed beyond Uranus would have been regarded as an interesting statement which remained to be proved.

Another case in which the existence of an unknown mass was suspected before the body itself was seen is that of the Dog Star, Sirius. This star-the brightest in the heavens-is moving through space, as, indeed, all stars are, but its change of position is not uniformly in one direction. The difference is very slight, and requires good instrumental means to detect it, but it could not be disregarded when once it had been established, and an explanation had to be found for it. This was given by Bessel in 1844, who suggested that a dark body near Sirius was exerting an influence upon its movements. Referring to the matter in a letter to Sir John Herschel, he wrote, "Light is no real property of mass. The existence of numberless visible stars can have nothing against the existence of numberless invisible stars,” and upon this idea he based the opinion that Sirius was a double star consisting of an invisible body as well as the visible one, the two forming a couple united by the bonds of gravitation. At that time it seemed scarcely credible that there were stars which could not be seen as well as those revealed by the telescope, but Bessel's belief has since received ample justification. Twenty years after he had given expression to it a faint star was discovered near Sirius, and it proved to be the body which causes the bright star to swerve from a straight path.

Stars with bright companions were discovered by Sir William Herschel towards the end of the eighteenth century, and their majestic march around one another was recorded. Many of these twin suns are nearly equal in brightness, but in most cases the two stars are badly matched in regard to visual appearance, a brilliant star

often being joined by the force of mutual attraction to one several degrees fainter. In the star Sirius this difference is accentuated to a noteworthy amount; for while it is the brightest object in the stellar universe, its companion is so faint that it can be seen only by using telescopes of great light-grasping power.

There is really nothing strange in this disparity when the matter is logically considered. Why should we measure mass in the universe by what we are able to detect with our limited optical sense? To a blind man, or to an eye capable of seeing everything, the brilliant star Sirius would not seem so overpoweringly great by the side of the dark heavy body to which it is united; and if mortals possessed a sense capable of being affected in proportion to substance, as the sense of sight is by luminosity, the dark body would have been noticed as soon as man turned his face towards the skies. For

though the faint star now seen to accompany Sirius would need to have its brightness increased twelve thousand times to equal the brilliancy of that gem of the sky, it is nearly half as heavy when measured by the standard of mass.

There are other cases in which the companions of bright stars are very faint, but heavy out of all proportion to their brightness. As with Sirius, the astronomer Bessel expressed the conviction that Procyon, which rises about half an hour before it, had a dark companion, disturbing its movements. Half a century later, in 1896, this companion was detected by Prof. Schaeberle, so we have here another example of a body known to exist long before it was seen.

The difference between mass and luminosity accounts for the interval that elapsed between prediction and discovery of each of the cases mentioned. The com