The form of the spiral nebulæ proclaims their motion, but one of its particular features discloses more. For it implies the past cause which set this motion going. A distinctive detail of these spirals, which so far as we know is shared by all of them, are the two arms which leave the centre from diametrically opposite sides. This indicates that the outward driving force acted only in two places, the one the antipodes of the other. Now what kind of force is capable of this peculiar effect? If we think of the matter, we shall realize that tidal action would produce just this result. We see it daily in the case of the Moon; when it is high tide in the open ocean hereabouts, it is high tide also at the opposite end of the Earth. The reason is that the tide-raising body pulls the fluid nearest it more strongly than it pulls the Earth as a whole, and pulls the Earth as a whole more than it pulls the fluid at the opposite extremity.
Suppose, now, a stranger to approach a body in space near enough; it will inevitably raise tides in the other's mass, and if the approach be very close, the tides will be so great as to tear the body in pieces along the line due to their action; that is, parts of the body will be separated from the main mass in two antipodal directions. This is precisely what we see in the spiral nebula. Nor is there any other action that we know of which would thus handle the body. If it were to disintegrate under
