134 



KNOWLEDGE. 



[July 1, 1891. 



yeai'. The year of century only must be used. For 

 other months arrange and number them as follows : — 

 March April 



i: 



August September 



1 2; 



January February 



2 3J 



Add the number for the month to that foimd for March, 

 plus 3 when marked thus J, reject sevens; remamder is 

 date of first Sunday in given month, from which other 

 days may be found. Note. — January and February are 

 the last two months of preceding year ; also, that the 

 numbers m second and fourth columns, marked J, are 

 30-day months. 



The above is for present century only. For next century 

 add 2 ; for last centiu'y reduce by 2, and generally for any 

 century, new style, divide centimes by 4 ; twice remainder 

 plus 3, throwing out sevens, gives number to be added. 



Old style centuries : To centuries add 5, or subtract 2, 

 and reject sevens. 



Examples. 1891. Next following leap year is 92, half 

 is 46 ; being third after leap year, deduct 3 ; dinde by 7, 

 remainder is l = date of first Sunday in March. April is 

 1 + 1+3 = 5; May, 1 + 2=3; June, 1+3 + 3=7; Decem- 

 ber, 1 + 5 = 6 ; February, '92, 1+3 + 3 = 7, &c., &c. 



A.D. 1, O.S. Centuries=0 + 5=5; year=2— 1 = 1 ; 

 5 + 1 = 6, date of Sunday in March. 



1815, Jime. Centuries=0; year is 8 — 3 + 3 + 3=11 + 

 7= 18th, a Sunday. 



Yours truly, 



C. Lund, Ilkley. 



To the KiUtor of Knowledge. 



SiK, — I was very pleased with, and greatly interested 

 in, Mr. Hutchinson's article on " The Cause of Volcanic 

 Action," which appeared in the current month's issue of 

 Knowledge. But there is one point upon which I should 

 feel extremely obliged if Mr. Hutchinson would give a little 

 further information. On page 106 he says, " A much more 

 promising explanation is that there are below the crust of 

 the earth large masses of highly heated rock, kept solid by 

 the enormous pressure of overlying rocks ; and when earth- 

 movements take place within the crust, such as the up- 

 heaving of a mountain chain, taking off some of the weight, 

 the balance of jjressure is no longer maintained, and so 

 the highly heated rock runs off in a liquid state, and finds 

 its way to the surface, producing volcanic action." I 

 should like to ask, am I right in assuming that the above 

 explanation refers only to the original formation of vol- 

 canos, and if so, what is the disturbing cause m the case 

 of Vesuvius, Etna, and other mountains, which have longer 

 or shorter periods of quiescence, and when active are not 

 associated (as far as I am aware) with earth-movements 

 such as the upheaval of mountain chains '? In the theory 

 propoimded by Bisohoft' (who, I thmk, believed the -centre 

 of the earth to be in a molten state), water is the great 

 factor as a disturbing cause. It also plays a very great 

 part in the theory (chemical changes) propounded by Su- 

 Humphry Davy, who held the opposite view with regard 

 to the earth's interior. Hoping, sir, that you wUl kindly 

 find a corner for this request in your next issue, 

 I remain, yours faithfully, 



H. Chkistopher. 



[A scientific theory which is " not proven " must neces- 

 sarily leave room for some diiSculties. I thmk it was 

 Wellington who said the best general was the one who 

 made the fewest mistakes. So with scientific theories — 



that one is the best which presents the fewest difficulties. 

 The question raised by Mr. Christopher is not easUy an- 

 swered. The explanation of volcanic action referred to in 

 my paper applies to the original formation of volcanos, 

 and explains their association with mountain chains ; but, 

 at the same time, I conceive that it also applies to the 

 case of Vesu\-ius and other volcanos now in eruption, 

 which do not appear to him to be associated with the up- 

 heaval of mountain chains. But in reality they are so 

 associated. The upheaval of the mountain chain was the 

 cause which brought them into existence, and, ha\ang 

 once begun their activities, they go on for a long period— 

 probably until the balance of pressure below has been 

 restored by the creeping of heated rocky matter in that 

 direction. Vesu\-ius will go on as long as such matter is 

 slowly impelled to its neighbourhood, just as water wiU 

 squirt out of an indiarubber ball as long as you squeeze it. 

 When other earth-movements start somewhere else the 

 movement may take place towards some other Ime of up- 

 heaval which at present does not exist. This does not 

 necessarily imply the presence of molten rock below, but 

 only of rock sufficiently heated and viscous to creep along 

 as clay or putty would imder pressure. I cannot believe 

 that water is the disturbing cause, for it must be present 

 in considerable quantity in all subterranean regions. 

 Lastly, a very little disturbance of the earth's crust may 

 suffice to alter the balance of pressure below. — H. N. 

 Hutchinson. T 



To the Editor of Knowledge. 

 SiE, — No doubt most of your readers have been favoured, 

 at some period of their Ufe, with a proof that one is equal 

 to two. Perhaps the enclosed view of the matter may be 

 equally convincing. 



Yours faithfully, 



R. Chabtres. 



Let B E C D be a section of a thin spherical shell, and 

 A B C a section of a cone touching the shell. 



It is easily seen that the attraction of the lower part 

 B D C on A is exactlyj equal to that of the upper part 

 BEG. Therefore, the attraction of the whole shell will 



