74 



SCIENCE-GOSSIP. 



Rhynchites pubescens, several out of oaks ; Otiorrhychus 

 picipes, several from hawthorn ; Strophosomus coryli, 

 common on anything but hazel ; Hylobrius abictis, 

 one ; Erirrhinus tertrix, two from aspen ; E. 

 maculatus, several from aspen ; Balaninus glandium, 

 one beaten from oak ; Diptera — Bombylins major, one 

 in a most dilapidated condition, having half of both 

 wings gone and very ragged pubescence; Hemiptera- 



heteroptera — Acanthosoma, three specimens of a 

 species beaten from hawthorn. A yellow species 

 was common on oaks, and a fine green capo id was 

 beaten from hawthorn at Rhinefields ; Hymenop- 

 tera ; Tenthredinidae — Tenthredo metadata, one 

 specimen, taken bj' Mr. Harvey near Brocken- 

 hurst. 



Lawrence J. Tremayne, Hon. Secretary. 



THE PHYSICAL STATE OF MARS. 

 By Felix Oswald, B.A. (Lond.) 



/~\NE of the most interesting points of connection 

 ^^^ between geology and astronomy consists in 

 the comparison of the present physical state of 

 our globe with that of the nearest members of the 

 solar system. Unfortunately such a comparison is 

 necessarily restricted, since the majority of the 

 planets are shrouded in dense atmospheres, apart 

 from the consideration of the vast distances 

 which lie between us and them. Mars and our 

 moon are the only two of our neighbours which 

 offer their entire surface to the scrutiny of our 

 telescopes, for it is but seldom that any part of 

 Venus is uncovered by clouds. 



The terrestrial vibrations due to the neighbour- 

 hood of large towns, as well as the density and 

 frequent obscurity of our own atmosphere, have 

 also proved to be serious drawbacks to the 

 observation of minute details. It has, however, 

 been latterly recognized that pure, steady air and 

 a lofty, isolated situation offer greater advantages 

 to an observer than the largest telescope placed in 

 such a locality as the Greenwich Observatory. 



The remarkable work done within recent 

 years under the most favourable atmospheric 

 conditions, by keen observers such as Professor 

 Barnard at the Lick Observatory, Professor 

 Pickering at Arequipa, and more especially Mr. 

 Lowell in Arizona, (*) has added enormously to our 

 knowledge of Mars and our moon. In both cases 

 we have to deal with a later stage of physical 

 development than that existing on our globe ; but 

 while the moon is evidently a dead world, Mars is 

 an example of vital but mature old age. In the 

 case of the moon, the distinctness with which the 

 physical features can be discerned is due not 

 merely to its close proximity, but above all to the 

 non-existence of an atmosphere ; in the case of 

 Mars (some forty million miles distant even when 

 nearest to us) it is due to the extreme tenuity of 

 the gaseous envelope, which is only one-seventh as 

 dense as ours, and to the consequent absence of 

 clouds, except to a very slight extent at sunrise 

 and sunset. The manner in which the presence of 

 clouds depends on the density of the atmosphere 

 (l) " Mars." By Percival Lowell. Longmans, 1896. 



may be readily noticed on our world, where the 

 lowest clouds are the largest and thickest, while 

 the highest consist of the delicate, fleecy cirrus or 

 "mackerel-sky," composed of ice-crystals, and 

 existing at a height of about five miles. Now our 

 atmosphere even at this altitude is denser than 

 that at the surface of Mars, so that this circum- 

 stance alone affords a ready explanation of its 

 cloudlessness. 



Although the Martian atmosphere is so much 

 thinner and rarer than ours, owing chiefly to 

 gravity being less, yet it is probably similar in 

 composition, as shown by Dr. Stoney's ingenious 

 deductions from the molecular theory of gases. It 

 is in accordance with this theory that any gas 

 consists of millions and millions of similar mole- 

 cules moving unceasingly in all directions with a 

 high velocity varying only with the nature of the 

 gas ; thus the molecules of hydrogen, the lightest 

 gas, have been shown to possess a maximum velocity 

 of over seven miles a second at 0° C. Now if a 

 projectile be shot vertically upward from the earth 

 it would travel away into space with a constant 

 velocity, never to return unless the attractive force 

 of the earth is sufficient to drag it back again. 

 This attractive force, called gravity, depends on 

 the mass of the earth, and therefore the earth can 

 only control projectiles endowed with a definite 

 limiting speed termed the " critical velocity," which 

 in the case of the earth is about 69 miles a second. 

 Hence if we know the mass of a planet we can at once 

 determine the various gases which are likely to be 

 present in its atmosphere. In this way Mr. Lowell 

 shows that we can satisfactorily account for the 

 absence of free hydrogen from our atmosphere, 

 since the maximum molecular velocity of hydrogen 

 is greater than the critical velocity of projectiles 

 with regard to the earth. On the other hand, 

 since the molecules of water-vapour, nitrogen, 

 oxygen, and carbon-dioxide possess a lower velocity 

 (2j, 2, i-8 and r6 miles a second respectively), 

 they have never been able to escape into space. 

 The moon has no atmosphere at all, because it can 

 only restrain molecules with a velocity of not more 

 than li miles a second, which is less than the 



