Apeil 2, 1894.] 



KNOWLEDGE. 



85 



to which the lunar crust has been subjected, If there had 

 originally been such a bridecake-like stratification, it 

 must long ago have been disturbed by the action of the 

 forces -which have built up and levelled again gigantic 

 lunar craters and mountain chains. 



The steep precipitous clifi's and tall pinnacles of rock 

 which are to be seen on so many parts of the moon are 

 standing evidence that the moon has a gaseous envelope 

 around it sufficiently dense to protect such rocky pinnacles 

 from being levelled by meteoric bombardment, for the 

 moon must encounter as many meteoric particles in its 

 annual journey round the sun as a similar area of the 

 earth's atmosphere encounters. But not a millionth per 

 cent, of the meteoric bodies encountered by the earth's 

 atmosphere reach the earth's surface ; they are, in nearly 

 every case, entirely driven into vapour before they reach 

 a height of fifty miles above the earth's surface. If the 

 lower hundred miles of the earth's atmosphere were con- 

 densed or done away with, it is evident that the earth 

 would be subjected to a very serious bombardment, for 

 even a particle as large as a mustard seed, projected with 

 a velocity of eighteen miles a second against a cliff face, 

 would bring down a portion of rock much larger than 

 itself, probably shattering the rock for a considerable 

 distance round the place of impact ; for we know that the 

 force of a blow increases proportionally to the square of 

 the velocity of the projectile, and the damage done by 

 meteors meeting the earth with a motion in a contrary 

 direction to the earth's motion round the sun would be 

 even more considerable. 



Such considerations seem to render it probable that the 

 moon has an atmosphere of some sort which acts as a pro- 

 tecting shield, or otherwise, in the course of geologic ages, 

 cUffs and pinnacles would have been shattered, and they 

 could not have retained their precipitous forms. 



Most of the meteors which we see consumed in our 

 atmosphere become visible at a height of less than 

 one himdred miles, and they are generally entirely driven 

 into vapour before they reach a height of fifty miles above 

 the sea level. Such an atmosphere as surrounds the earth 

 at a height of fifty miles above the sea level would con- 

 sequently be sufficiently dense to enable the moon to 

 preserve the sharp outlines of its geologic features. If we 

 assume that the earth's atmosphere continues to halve in 

 density with every increase of three and a half miles by 

 which we rise above the earth, the density of our atmos- 

 phere at a height of fifty miles would be about one fifteen 

 thousandth part of the density at the sea level. 



The observations with regard to lunar occultations and 

 solar eclipses, referred to above, enable us to say that the 

 moon certainly is not surrounded by an atmosphere of 

 similar material to our own, which is one two-thousandth 

 part as dense as the earth's atmosphere at the sea level. 



But it may very possibly be surrounded by an atmos- 

 phere which is one ten-thousandth part as dense. Assuming 

 that it is surrounded by such an atmosphere, the 

 absence of white caps about the lunar poles would seem to 

 show that the cold, even in the polar regions, is not suffi- 

 ciently intense to freeze the lunar atmosphere. If the 

 lunar air is similar in chemical composition to our own, 

 this would enable us to affirm that the temperature of the 

 lunar poles has not yet fallen below about -180° Centigrade. 

 It is evident that at some period, the geologic record of 

 which is not yet effaced, there were considerable volcanic 

 displays at the lunar poles, for both the northern and 

 southern lunar polar regions are thickly crowded with 

 crater rings — indeed, the volcanic activity at the lunar 

 poles seems to have been greater than in the lunar equa- 

 torial regions. 



Stttncc Notes. 



Prof. Dewar, in a recent lecture at the Eoyal Institution 

 on the scientific use of liquid air, and the study of the 

 properties of matter under very low temperatures, demon- 

 strated that in low temperatures the tensile strain of 

 metals is stronger, and their colour also is less brilliant. 



Some interesting details have been published in the 

 transactions of tin- Texas Acadoiii/ of Science, concerning 

 rain-making and the rain-makers. For the problem of 

 overcoming the severe drought in such districts as the 

 United States, or New South Wales, has, at irregularly 

 recurring intervals, engaged the attention both of men of 

 science and various impostors, who have endeavoured 

 to persuade the Government to make trial of their 

 schemes. The problem that has to be solved is the cool- 

 ing of the upper layers of the air to such an extent that 

 some of the moisture contained in it shall be precipitated. 

 One of the theories on which several rain-makers based 

 their suggestions was that if a rapidly ascending current 

 of air could be produced, on reaching a great height this 

 would expand under the conditions of diminished pressure, 

 and in expanding would cool, and so precipitate some of its 

 moisture. But the production of this ascending current is 

 in itself the first difficulty. The scheme that Prof. Espy 

 proposed in 1837 was to kindle great fires, thus producing 

 a rising current of hot air. It would be better for the 

 success of the scheme if the air could be induced to rise 

 before first heating it, as the object of the theorists is to 

 produce cooUng in the upper strata. 



It is a well-known fact that a shock of some kind given 

 to a supersaturated atmosphere will sometimes precipi- 

 tate the excess of moisture held in suspension, and it 

 seems to be some distortion of this fact that has actuated 

 some of the schemes of the rain-makers ; for they appear 

 to have thought that a severe shaking was all that was 

 necessary to induce the atmosphere to render up its 

 aqueous vapour. Mr. Powers, of Wisconsin, published in 

 1870 an ingenious collection of random statistics entitled 

 " War and the Weather," in which he endeavoured to 

 prove that battles were followed by heavy rainfall. The 

 same idea moved another rain-maker, and having obtained 

 a Government grant of a considerable sum, he proceeded to 

 make war upon the elements in true military style. Ground 

 explosives were fired off at some advancing rain-clouds, 

 then balloons, charged with explosives, were fired inside 

 them, shells were projected at them, but the clouds sailed 

 away in unrufHed serenity, regardless of the rain-maker and 

 his challenge. 



A more scientific scheme was afterwards patented, 

 which consisted in freeing liquefied carbonic acid in 

 the air ; this liquid, by rapid vaporization, producing 

 great cold. But as a frequent expedient, this plan is 

 marred by the fact that the cost of one inch of rainfall 

 over a square mile would be four hundred thousand dollars. 



Some curious mechanical devices have been seriously 

 proposed for rain-making, such as Mr. Pitkin's large sheet 

 of canvas, which was to be hung in the air with the object 

 of deflecting upwards warm air currents into a colder strata. 

 Mr. Astor's invention is a tower, up which air is to be 

 piimped by an engine, such a tower having presumably to 

 be about the height of the Eiffel tower, and therefore 

 similar in cost. The engine too would probably require 

 much more water than the whole apparatus would conjure 

 out of the atmosphere. 



