204 



KNOWLEDGE & SCIENTIFIC NEWS. 



[August, 1905. 



Acetylene Qls SLn. 

 Explosive. 



Some interesting researches have recently been made 

 by M. Grehant on explosive mixtures of air and acety- 

 lene and corresponding mixtures of air and oil gas. The 

 tests were made in tubes of 50 cubic centimetres for the 

 acetylene and of 90 cubic centimetres for the oil gas. 

 The mixtures were exploded by an electrically-heated 

 incandescent wire. The following are the results 

 obtained : — 



Volume Volume Perceniage 



of gas. of 



of gas. 

 50 o 

 333 



With acetylene. 



Btims with smoky flame. 



Ditto. 

 Detonates with deposi- 

 tion of carbon. 

 Stronger detonation with- 

 out depcsition. 

 Strong detonation. 



Ditto. 

 Very strong detonation 



Ditto. 



Ditto. 

 Strong detonation. 



Ditto. 



Ditto. 

 Less strong. 



Ditto. 

 Feeble detonation. 

 Very feeble detonation. 

 Bums without detonation. 



With Oil-gas. 

 Does not bum. 



Ditto. 

 Feeble detonation. 

 Stronger detonation. 

 Strong detonation. 



Ditto. 

 Less strong. 



Ditto. 

 Feeble detonation. 



Ditto. 

 Very feeble detonation. 

 Does not ignite. 



Ditto. 



Ditto. 



Ditto. 



Ditto. 



Ditto. 



These results show that the detonations obtained with 

 acetylene are more violent than those with oil-gas, but 

 that, nevertheless, the acetylene is less dangerous than 

 oil-gas. 



COR^RESPONDENCE. 



The Visibility of Planets in Daylight. 



Sirs, — Some of your readers may be interested in knowing 

 that it is quite possible for them to see the planet Venus in 

 broad daylight, with the naked eye, and no apparatus of any 

 sort. I have for the last six months or so constantly seen 

 her shining high up in the heavens in the blazing light of a 

 South African sun at mid-day. It merely requires one to take 

 a rough measurement on a stick, and one evening, about sun- 

 set, to see how far she is from the sun, and the direction. You 

 cam then, next day, discover her without opera glasses or any 

 other help. Of course, at first it is extremely hard to find the 

 planet, but after a little practice the eye picks it up easily. I 

 have found it on a cloudless sunny day while riding along the 

 veld. I have also succeeded in seeing Jupiter in full sunlight 

 in the same way, and lately I have, owing to this practice, 

 managed to find Sirius and Canopus with the sun some 

 way above the horizon, and shining brightly. It is not only 

 owing to the clear atmosphere of this country, as I have 

 found Venus on a somewhat hazy day, certainly far less clear 

 than one often gets it in England. It would be interesting 

 also to know if any of your readers have seen the waning moon 

 within 40 hours or so of new moon. By careful search I have 

 succeeded in seeing it about that time from new moon, in mid- 

 day in a cloudless sky, nearly overhead. As she is so exceed- 

 ingly thin and close to the sun, you can imagine how hard it 

 is to find her with the naked eye. Those novelists or poets 

 who have been decried by the critics for making the crescent 

 moon shine overhead, will now be able to refer to my state- 

 ment in verification of their words. 



I am, yours truly, 



T. B. Hlathwayt. 



Kokstad, E. Griqualand, S. Africa, 

 April 29, KJ05. 



[As regards Venus, it is well known that this planet " Is so brilliant 

 that there is no real difficulty in seeing her with the naked eye in full 

 sunshine, or indeed at high noon." — Maundir, " Aitronomy without 

 a Tcltscuft," p. 147. — Ed ] 



London Fog acnd Frost. 



Sirs, — In a paper lately read before the Koyal Meteorologi- 

 cal Society, the author, Mr. F. J. Brodie, gave some useful 

 tabulated data of fog observed in London during twenty years, 

 and came to the sanguine conclusion that the great decrease 

 of fog in recent years points to a victory over the fiend by 

 smoke abatement in various ways. 



Without aftirming that there is no improvement from this 

 cause, or questioning the obvious wisdom of efforts to do away 

 with smoke, I cannot but think that climatic influences are the 

 chief factor in the improvement observed. 



If we combine Mr. Brodic's figures for autumn and winter, 

 andsmoothe the scries by sums of five (i.e., adding the first five. 



197/ 'i. '7 'go '3 '6 "9 '9?- "y '9 O) k^ 



A.— Smoothed curve of Pog-days. 

 15.— ,, ,, l-rost-day^ 



then the second to the sixth, and so on), we get the curve A ; 

 and doing the same with the totals of frost days in winter 

 seasons at Greenwich, we have the curve B. 



This is obviously a rough comparison, and remembering also 

 the uncertainty attaching to fog determinations, we should not 

 look for very exact correspondence in these curves ; but there 

 is general agreement, and, in particular, while the fog curve 

 shows a long general decline from about 18S9, the frost curve 

 does the same. 



If we get up again to the 1889 level of frost, will the fog curve 

 fall short of Us level for that year ? That remains to be seen ; 

 and before giving rein to the triumphant spirit, we had better 

 first see, perhaps ! 



I am yours, &c., 



Alex. B. MacDowall. 



ASTR.ONOMICAL. 



By Charles P. Blti.lk, A.K.C.Sc. (L.md.), I'.R.P.S. 



The Canals of Mars Photographed. 



A snf)KT time back thf: important news was tclegr.iphed from 

 Lowell Observatory that thu much-criticised markings on 

 the Martian surface had been successfully photf)graphed 

 by Mr. Lampland, and the last circular from the observatory 

 not only confirms this, but contains an actual print from the 

 negative showing the markings. 



Many attempts have been made at the Lowell Observatory 

 at Flagstaff, Arizona, to photograph the canals of Mars, com- 

 mencing with the success of Mr. Douglass in ivoi, when, by 



