THE BOUNDARIES OF ASTRONOMY. 245 



tinguish the parts of their orbits which we see from actual parabolas. 

 Nor do the directions iu which the comets move exhibit any uniform- 

 ity ; some move round the sun in one direction, some move in the op- 

 posite direction. Even the planes which contain the orbits of the 

 comets are totally different from each other. Instead of being in- 

 clined at only a very few degrees to their mean position, the planes of 

 the comets hardly follow any common law ; they are inclined at all 

 sorts of directions. In no respect do the comets obey those princi- 

 ples which are necessary to prevent constitutional disorder in the plan- 

 etary system. The consequences of this are obvious, and unfortunate 

 in the highest degree for the comets. A comet possesses no secu- 

 rity for the undisturbed enjoyment of its orbit. Not to mention the 

 risk of actual collision with the planets, there are other ways in which 

 the path of a comet may experience enormously great changes by the 

 disturbances which the planets are capable of producing. How is it 

 that the system has been able to tolerate the vagaries of comets for 

 so many ages ? Solely because the comets, though capable of suffer- 

 ing from perturbations, are practically incapable of producing any 

 perturbations on the planets. The efficiency of a body in producing 

 perturbations depends upon the mass of the body. Now, all we have 

 hitherto seen with regard to comets tends to show that the masses of 

 comets are extremely small. Attempts have been made to measure 

 the masses of comets. Those attempts have always failed. They 

 have failed because the scales in which we have attempted to weigh 

 the comets have been too coarse to weigh anything of the almost spir- 

 itual texture of a comet. It is unnecessary to go as far as some have 

 done, and to say that the weight of a large comet may be only a few 

 pounds or a few ounces. It might be more reasonable to suppose that 

 the weight of a large comet was thousands of tons, though even thou- 

 sands of tons would be far too small a weight to admit of being meas- 

 ured by the very coarse balance which is at our disposal. 



The enduring stability of the planetary system is thus seen to be 

 compatible with the existence of comets solely because comets fulfill 

 the condition of being almost imponderable in comparison with the 

 m ighty masses of the planetary system. The very existence of our 

 planetary system is a proof of the doctrine that the masses of the 

 comets are but small. Indeed, to those who will duly weigh the mat- 

 ter, it will probably appear that this negative evidence as to the mass 

 of the comets is more satisfactory than the results of any of the more 

 direct attempts to place the comets in the weighing-scales. If we re- 

 state the circumstances of the solar system, and if we include the com- 

 ets in our view, it will appear how seriously the existence of the com- 

 ets affects the validity of the argument in favor of the nebular 

 hypothesis which is derived from the uniformity in the directions 

 of the planetary movements. If we include the whole host of minor 

 planets, we have for the population of the solar system something 



