314 



♦ KNOWLEDGE 



[May 25, 1883. 



granting freoly at the outset that I am " a pleasant and, 

 even to a considerable degree, a profitable figure in contem- 

 porary life " (imagine a man being a pleasant and profitable 

 figure !), cannot inveigle me into a verbatim reading. Yet, 

 unless I mistake, I have reason for knowing that, when 

 decently paid, the writer of this rubbish can do much 

 better. 



" Let Knowledge grow from more to more." — Alfked Tennyson. 



ilfttns! to tt)t eiiitoi-. 



Only a small proportion of Letters received can possibly be in- 

 eerted. Correspondents must not ie offended, therefore, should their 

 letters not appear. 



All Editorial communications should he addressed to the Editor of 

 Knowledge ; all Business communications to the Publishers, at the 

 Office, 74, Great Queen-street, W.C. If this is not attended to, 



DELAYS ARISE FOR WHICH THE EDITOR IS NOT RESPONSIBLE. 



All Remittances, Cheques, and Post Office Orders should be made 

 payable to Messes. Wyman & Sons. 



The Editor is not responsible for the opinions of correspondents. 



No COMMUNICATIONS ARE ANSWERED BY POST, EVEN THOUGH STAMPED 

 AND DIRECTED ENVELOPE BE ENCLOSED. 



TO DESTROY APHIDES. 

 [825] — The following will relieve yonr roses ot aphides : — On 

 quassia chips pour boiling water. When cold, strain, and mix with 

 solution of soft-soap ; use cold. They are obtained from a large 

 forest tree of Jamaica, and other West Indian islands. Its pure, 

 bitter principles depend on the presence of a crystalline quality, 

 called quassite, or qnassiue. It is employed as a tonic and febri- 

 fnge. A solution acts as a narcotic poison on flies and otherinsects, 

 and can be employed as an anthelmintic. Law stopped its being 

 used as a substitute for hops. The tree is Quassia (Picrcena 

 excelsa), and attains a height of nearly 100 feet ; and is named after 

 a negro (Quassy), who first discovered its virtues. 



John Ales. Ollakd, F.R.M.S. 



SIWALIK HILLS. 

 [826] — The correspondent who seeks information regarding the 

 tertiary deposits of the Siwalik Hills, in India, will find what he 

 requires in Medlicott and Blandford's " Geology of India." 



C. Carcs-Wilson. 



FLIGHT OP VERTICAL MISSILE. 



[827] — A correspondent in Adelaide asks this question : — If a 

 perfectly spherical ball bo shot vertically upwards from the earth's 

 equator, and in vacuo, will it fall to the eastward or westward of 

 the point of projection ? Thus, if we take the upward velocity at 

 1,000 ft. per second, and knowing the equatorial velocity of the 

 earth — required, the precise amount of deviation, if any. 



W. P. L. 



[Three replies were obtained by W. P. L. and his friends — to wit, 

 that there would be a westerly deviation of 0'25 ft., 71 ft. (!), 

 282 ft. (! !). I leave the problem as an exorcise to readers, but it is 

 hardly necessary to say the two larger deviations are quite wrong. — 



EDISON ELECTRIC LIGHT COMPANY. 

 [828] — It is a truism that error dies hard. The paragraph on 

 page 230 of your issue ot this date, relative to the cost of tho 

 Edison installation at tho House of Commons, is an example in 

 point. The Daily jYcics, I believe, originally promulgated this, 

 and it has since gone tho round of the weeklies. As you m.ay have 

 observed, from the answer of the First Commissioner of Works to 

 a question by Lord Randolph Churchill in the House lately, the 

 contract with tho Edison Company was for tho lighting of the 

 library and dining-rooms during the current session at a rate some- 



what e.xceeding the cost of gas, or rather over £100. As regards 

 tho alleged cost of making the installation, while I do not writ© 

 officially, I may say that the sum named — £2,000 — is much in 

 excess of the actual amonnt expended. 



COBBESPONDENT. 



A DISCOUNT DODGE. 



[829]—" G. M.," in letter 812, p.ige 2C9, falls into an error 

 regarding easy method of calculating discounts. I found the 

 method in an old arithmetic, and have used it for many years. 

 This is tho rule as given in the arithmetic I refer to : — Multiply 

 tho principal by double the rate of discount. With the product 

 cut off tho last figure to tho right in the pounds, the figures to tho 

 loft will then represent shillings, and the figure to the right will 

 represent pence and the fifths of a penny. Take 3| per cent, dis- 

 count on the sum of £187. 10s. 

 By the ordinaiy 

 method. 

 £. s. d. 



187 10 



3,00 = £6 11 3 



The last method will be found to work true, no matter what 

 parts of a pound there may be in the principal. For instance, — 

 £123. 4s. Id. at 5^ percent., say £123. 4s. Id. x U = £135,5. 4s. lid. 



a fifth added 1 114 



[Readers will find it a 

 comes about — R. P.] 



6 5 10^ 

 = £6. 15s. G,Vad. 

 T. J. K. 



study to note how this result 



EASY MULTIPLYING. 



[830] — I beg to call your attention to another easy multiplication. 

 About eight years ago, an American doctor taught me a simple 

 method to get square powers of these nine numbers, viz., 15, 25, 35, 

 45, 55, 65, 75, 85, and 95. Rule. — Prefix the product of the left- 

 hand figure, multiplied by another higher by one than itself, to the 

 square of the 5. 

 Examples : — 



35x35 3x4 = 12 65x65 6x7 = 42 



5x5= 23 5x5= 25 



1225 



4225 



The following is a method found out by myself. By this I can 

 multiply any two numbers under 100, providing that either left or 

 right-hand figures are alike. For instance : 

 24x34 =(20x30) = 600 

 (20-1-30) 4=200 

 4 X 4= 16 

 816 

 61x67 = 60x60 = 3600 

 (1-H7) 60= 480 



1x7 = 7 



4087 

 This method is not unlike that of Mr. J. Smith, but, I think, a 

 little easier. If I write down both multiplicand and multiplier, thus : 



51 



— result ot which will be so, 

 58 



58 

 2958 



I a moment. 



EXTRACTING SQUARE ROOT. 



[831] — Perhaps the following mode of extracting square roots 

 may interest some of the readers of Knowledge. I can't say mncb 

 for its practical utility, but I have now and then found it handy 

 when I have not had a logarithm book available. It, of course. 



