KNo\vLi:i)(-i:. 



SkI'TEMBKK, 1912. 



29tli, \')\2. \Vc count backward 3 days from Monday to 

 Friday, answer. 



This method can be readily used when the period is short. 



Sixth problem, by third method : — 



The next transit of N'cnus will be on June 8th, 2004. Find 

 the day of the week, knowing that June 8th, 1912, was 

 Saturday. 



Solution : — 



The period is 92 years. Both first .ind last are leap years. 

 The number of leap days is one fourth of 92 or 23. The sum 

 of 92 and 23 divided by 7 gives a remainder 3. Count 

 forward i days from Saturday to Tuesday, the answer. 



When it is retjuircd to know on what day of the week 

 February 29th, either was or will be, find the day of the week 

 for February 28th. of the same year, by one of the methods 

 herein described, then count forward one day. 



Seventh problem. 



Find on what day of the week February 29th, 3992, will be 



knowing that February 28th, 1912, was Wednesday. 



Solution : — 



The period is 2080 years. Both first and last are leap 

 years. The fourth of 2080 is 520. The years 21, 22, 23, 25, 

 26, 27, 29, 30, 31, ii, 34, 35, 37, 38 and 3900 will not be leap 

 years. There are 15 of these. Substract 15 from 520 and 

 find that there will be 505 leap days. The sum of 2080 and 

 505 divided by 7 gives 2 for remainder. Count forward 2 

 days from Wednesday, and find that February 28th, 3992, 

 will be Friday. Of course, February 29th, 3992, will be 

 Saturday, the answer. 



The third method is the best for general use. 



Should the reader prefer, he may find the number of leap 

 days between the two given dates by counting. For example, 

 in the fourth problem, he can see that between November 

 18th. 1883, and November ISth, 1912, were the following leap 

 days— February 29th, 1884, 1888, 1892, 1896, 1904, 1908 and 

 1912. There are 7 of these, as we have already found. 



NOTICES. 



THE DUNDEE MEETING OF THE BRITISH 

 ASSOCIATION. — The meeting of the British Association 

 which opens under the general presidency of Professor 

 Schafer at Dundee on September 4th, bids fair to be an 

 excellent and an enjoyable one. The city, as we can speak 

 from pleasurable experience, knows how to entertain its visitors 

 right royally, and one feature of the meeting will be the large 

 number of foreign guests. Judging, too, from the presidential 

 addresses to the Association as a whole, and to the various 

 sections, which we are privileged to read, but whose nature we 

 are not permitted to reveal until they have been delivered, 

 members old and new who go to Dundee will not regret 

 having made the journey. In this connection we have pleasure 

 in :eproducing for the benefit of our readers a time table of 

 the trains from Euston, which the London and North Western 

 Railway has kindly forwarded to us. 



Week-days. 



LT LD S Ssx 



am. p.m. p.m. p.m. 



London (Euston) ... dep. 10.5 2.0 8.0 11.45 



.\ A.s 



p.m. a.m. a.m. a.m. 



DUNDEK (West Station) arr. 8.50 1.15 6.45 9.52 



Sundays. 



London (Euston) 

 Dundee (West Station) 



s 



p.m 

 dep. 8.0 



a.m. a.m. 

 arr. 6.45 9.37 



p.m. 

 8.50 



p.m. 

 11.45 

 a.m. 

 9.52 



Notes : 

 LT — Luncheon and Tea Cars. .\ — Through Carriages to 



LD — Luncheon and Dining Cars. Dundee. 



S — Sleeping Cars attached. .\\ — -Through Carriages to 



S sx — Sleeping Cars attached, Dundee, e.\cept Saturday 



except Saturday Nights. Nights. 



Charge for sleeping berth 10s. in addition to first class fare, 



HOUSE FLIES. — Of recent years the harm which house 

 flies may do in the carrying of infection has been emphasised. 

 Some very careful experiments, which are described by Dr. 

 Graham-Smith in the current number of Bedrock, give a clear 

 idea as to how long after contamination the germs of disease 

 flies may continue to be dangerous. The typhoid bacillus may 

 remain alive in the intestines of the flies for at least six days 

 and flies can infect materials over which they walk for at 

 least two days. Bacilli which produce the symptoms of meat 

 poisoning behave in the same way. Tubercle bacilli can be 

 found in the intestines of flies ten days or more after infection. 

 In the case of germs which produce spores that can only be 

 killed with great difficulty, these may remain on the dead fly 

 for months or even years. It was also shown that flies which 

 feed on milk, or which tumble into it. are capable of infecting 

 it, and although under ordinary conditions in this country the 



adult fly seldom has the opportunity of feeding on materials 

 infected with disease-producing bacteria, yet the maggots are 

 often probably infected. Further researches are being made 

 into tlie cjucstion whether the flies which spring from these would 

 also be infected, and we await the results with much interest. 

 CLASSES IN PHOTOGRAPHY.— We have pleasure in 

 announcing that Mr. Edgar Senior's classes in Photography 

 for the autumn session will open at the following Polytechnics 

 on the dates specified: Battersea, Tuesday, October 1st; 

 South Western, Monday, September 23rd ; and Woolwich, 

 Wednesday. September 25th, 1912. 



THE OPTICAL CONVENTION. 1912.— With further 

 reference to new apparatus exhibited at the Optical Conven- 

 tion ("'Knowledge," August, 1912, page 291) we may refer 

 to that shown by Messrs. Newton & Co. Special mention 

 should be made of Cheshire's Optical Apparatus, never before 

 exhibited in the complete form in which it was shown at work 

 during the Convention. It consists of an optical bench which 

 can be mounted in the form of a Projection Spectroscope, 

 Polariscope or any similar apparatus. The Colour Projection 

 portion is probably the best method of showing the composi- 

 tion of white light at present devised, and consists of recom- 

 posing the spectrum on the screen by means of a convex lens. 

 In the spectrum itself is then placed a number of wedge prisms 

 cut slant-wise, which reflect portions of the recomposed beam 

 upwards and outwards so as to give a number of overlapping 

 colour discs. These discs change their tints as the wedge 

 prisms are moved along the spectrum, but they are always 

 complementary, and the overlapping portion in the centre is 

 alwaj's white. 



Another development of the same apparatus is the Model 

 Eye, also arranged for projection on the screen, the picture of 

 the retina itself being projected and the various defects due 

 to presbyopia, myopia and astigmatism demonstrated. 



.Another instrument shown was a cheap three and a half 

 inch astronomical telescope, of great optical perfection, on a 

 simple yet strong altazimuth stand. In addition to these 

 there was a full range of the firm's well known optical 

 apparatus. 



WHAT DETERMINES SEX.— To Scieiitia for May, 

 Professor Arthur Thomson contributes a critical essay 

 entitled, " What determines sex." He discusses five 

 difl'ercnt theories, and concludes by giving his o\ni view- 

 that the difterence between an ovum-producer and a sperm- 

 producer is fundamentally a difference in the balance of 

 chemical changes, that is, in the ratio of anabolic and 

 katabolic processes which may, of course, have its structural 

 expression in the relation of nucleo-plasni and cytoplasm. In 

 fact, he adheres to the thesis of his book, " The Evolution 

 of Sex," that the sex-difference is one expression of a 

 fundamental alternative in variation to be seen throughout 

 the world of life. 



