412 



KNOWLKDGE. 



OCTOBKR 



l')ll. 



THE VELOCITY Ol" LKtHT. 

 To the Editors of " Knowlkhgf,." 



Sirs. — A Hasli of " forUed " li^htiiinf,' is visible to the eye. 

 and seems to take an appreciable time to pass from the 

 thundercloud to the earth. As the human eye is said not to 

 be able to see anythinj,' that takes less than about one-tenth 

 of a second to pass tlie line of vision, a flash of " forked " 

 lixhtnint; must take more than one-tenth of a second to pass 

 from cloud to earth, a distance of a mile or two. « hich would 

 be equivalent to a velocity of about twenty miles per second. 



If this be so. is there not a very great retardation in the 

 velocity of light, at or near the level of the earth ? If light 

 travel at the average rate of one hundred and eighty-six 

 thousand miles per second, and so great a retardation is 

 caused by the density of the atmosphere near the earth's 

 surface, may not light travel at a very much greater rate in 

 space than one hundred and eighty-six thousand miles per 

 second ? 



The experiments and observations for the calculation of the 



vclocit\' of li'_;ht were made at or near the surface of the earth; 



.uid if the earth's atmosphere does in any way reduce the 



velocit\- of light, would it not be possible and advisable to 



recalculate that velocity from experiments and observations 



made at observatories situated from five thousand feet to 



fifteen thousand feet above sea level? ^ „ /-ttitil- 



Lr. K. Li 1 rSnrt. 



cll'sti:rs and nebulae. 



To tin- lutitors of " Knowledge." 

 Sirs, — Si'ptcmhcr nimiber, page J45. 1st cohmin : 



"Clusters 



and groups of stars . . . which seem to be connected 

 by a force acting far more powerfully and at greater 

 distances than gravitation." 



1 understand that gravitation h.d.inccs tlic members of the 

 Solar system uith each other, and inferred that it has a 

 similar effect in all other systems and on every system with 

 every other. Hut Mr. Henkel suggests some other force which 

 seems imnecessary. ^ ., 



QUERIES AND ANSWERS. 



Readers arc invited to send in Questions and to answer the Queries wliicli are printed here. 



QUESTIONS. 



51. GEOLOGY OE SOUTH DORSET.— Would one of 

 your readers give me U few facts about the Geology of South 

 Dorset — the strata, fossils and minerals. The region of 

 greatest interest is the Swanage neighbourhood. c tj i? 



bl. GENIUS AND ISOLATION.— I have a theory th.at 



social isolation in youth is one of the primary causes of 



genius. Can some one quote me proverbs and facts in 



the lives of such men as Shakespeare and Napoleon, tending 



to demonstrate this hypothesis ? ;, ,, ,, 



^>. H. K. 



53. STRANG!'" STARS. — Can any of your readers give me 

 particulars as to the three strange stars seen last month 

 on the right hand of the moon \\]\v\\ nrarlv at tlu' full in 

 the South-East. 



I am told they only appear once in one hundred years ; is 

 this so ? 



Some reference was made to these in a Scotch paper, but I 



have seen no mention in any of ours, or " Knowledge." 



The stars are not now to be seen. ,,, ,, 



Walter Hradshaw. 



REPLIES. 



45. HEAT AND VACUUM.— The loss of heat by a liquid 

 contained in an ordinary vessel is due mainly to conduction. 

 Defining heat (in the sense of a high temperature possessed 

 l)y a body relative to its surroundings), as a state of increased 

 molecular motion, cooling results from conduction by 

 surrounding matter. If, now. such conduction be almost 

 wholly prevented by a vacmmi jacket, the most potent cause 

 of loss is removed, and cooling proceeds by radiation. This 

 introduces the conception of radiant heat, which is quite 

 distinct. Radiant heat is a wave disturbance which is 

 radiated through space like light, and which produces a 

 condition of increased molecular motion (heat) in any absorb- 

 ing matter on which it falls. The spectral location of the 

 radiant heat rays is in the infra-red. The full efficiency of a 

 vacuum ve.ssel can be obtained by making the walls highly 



reflecting. ,, ,,t i-, 



Charles W. Raffetv. 



47. GRAVITY. — It is impossible for the atmosphere of our 

 earth or of any of the Planets to prevent bodies flying off into 

 space. If the absence of an atmosphere would mean that 

 heavenly bodies could not retain anything on their surfaces, 

 we might expect pieces of the Moon to be continually breaking 

 off'; for it is gener.ally admitted that the Moon is practically 

 devoid of an atmosphere. Of course, the resistance of the 

 atmosphere will always diminish the velocity of a moving 

 body, but this cannot have any connection with the point 

 raised by " Ignoramus," For instance, if we could fire a 

 bullet from the Earth with a velocity of about seven miles 

 per second, it would fly off into space, the Earth losing control 

 over it. This is based on the assumption that thei^e is 

 no atmosphere to hinder its motion ; the presence of the 

 atmosphere, however, would necessitate a velocity of projec- 

 tion greater than seven miles a second, in order that the bullet 

 might not return. Evidently, however, " Ignoramus " is not 

 considering such extreme cases as this, but seems to be under 

 the impression that bodies would rise spontaneously in the 

 absence of an atmosphere, and pass oft' into space. Not only 

 is this a false idea, but bodies would actually weigh more if 

 our atmosphere disappeared. The weight of one cubic foot 

 of air at sea level apd at normal temperature and pressure is 

 •0.S07 pounds. In acc6rd,-ince with the well-known law, 

 that bodies innnersed in a fluid lose a weight equal to the 

 weight of fluid displaced, a body of volume one cubic foot 

 would weigh -0807 pounds less in air at sea level than in a 

 vacuum. Evidently, then, there would be less tendency to fly 

 off, generally speaking, if we had no atmosphere. The simple 

 experiment of placing a few bodies under the receiver of an 

 air pump, and then producing a vacuum, will show that there 

 is no tendency for these bodies to fly off. 



Whatever explanations may be given of the causes of 

 Gravity, whether due to Etherial Tension or to any other 

 cause, the fact that such a force exists cannot be denied. It 

 surely cannot have been seriously advocated in the sermon 

 referred to, that the earth retained bodies on its surface by 

 atmospheric pressure without the force of Gravity ! 



Henry E. Crick. 



NOTICE. 



ARTIFICIAL RAIN.— In a little pamphlet under this title 

 Mr. Emilio Olsson describes a sprinkling apparatus for crops 

 which he claims will take the place of rain in times of drought, 

 and he gives an account also of some experiments carried out 



in the way of electrifying the water used. A certificate from 

 two engineers that the electrifying of the water reduced the 

 total h.udness and that it was able to kill locusts perfectly is 

 included. Mr. Olsson's address is 19, Southampton Buildings. 



