172 
THE GEOLOGIST. 
actly analogous to that held by the Chelonians amongst the more highly 
organized reptiles. Much careful investigation is necessary, and still more 
perfect specimens are required before this can be fully wrought out. 
It is right here also to state that to Mr. Mitchell belongs the merit of 
first discovering Pteraspis in our Scottish rocks, although it is only very 
recently that I was aware that he had procured and recognized fragments 
of this fish some time anterior to m}'- discovery of its remains. Believing 
our Scottish Pteraspis to be specifically distinct from the other species yet 
found, in a paper which I hope to have tlie honour of communicating at 
an early meeting of the Geological Society of London, noticing it along 
with some other Forfarshire fishes, I propose his name as a specific affix 
for it, and that it should be known as Pteraspis Mitchelli. 
I am, dear Sir, Yours ever truly, 
James Powkie. 
Heswalla, April^ ISG^. 
Spiral Planetary Orbits. 
Sir, — Your highly suggestive article on " Spiral Planetary Orbits '* 
{vide ' Geologist ' for March) gave rise to some ideas which may prove in- 
teresting to those of your readers who are partial to speculative inquiries. 
The generally accepted explanation of the planets' tran si atory motion is, 
that those bodies were projected once for allmio free space with great ve- 
locity, and that as they meet with no resistance they will always continue 
their course. 
The existence of free space here assumed, is, however, very doubtful, 
since we can hardly reconcile a perfect vacuum with the transmission 
through it of light and heat, for we know of no such thing as physical 
force existing independently of matter. But, as you have already shown, 
if matter does occupy space, then, however rarefied it may be, there must 
be resistance, friction, and consequently retardation of planetary motion. 
This slackening of the speed, by destroying the equilibrium of the centri- 
petal and centrifugal forces, would contract the orbits, and ultimately 
cause the planets to fall into the sun. The equilibrium of the two forces 
once destroyed, both the decrease of speed and the increase of attraction 
would tend to the same end, and the motion towards the sun would be so 
continuously and immensely accelerated that the final catastrophe would 
not perhaps be so far distant as might at first be imagined. 
To such a view of planets revolving in a plenum, without any supply of 
motive force but that first acquired, some difficulties present themselves, 
not the least of which is, that if the results of retarded planetary move- 
ments are expected to evince themselves in the future, they may also be 
looked for at present, as phenomena indicative of such retarded movements 
during time past ; for we know not, neither can we imagine, what proportion 
the past bears to the future. 
But can we discern any such phenomena? Not in the planetary cir- 
cuits, for the centripetal and centrifugal forces still appear to balance each 
other, their equilibrium remains undisturbed, and we do not find that those 
planets nearer the sun have a decreased orbital velocity. On the contrary, 
for " the angular velocity of a planet's movement in its circuit is inversely 
as the square of its distance from the sun." 
How, then, can we reconcile the continued regularity of planetary mo- 
tion w ith the existence of a resisting medium in space ? Does it not ap- 
pear as though we should have to discard the " projected once for all " 
