€|)e ^ojjular Science ^ttai 



AND 



BOSTON JOURNAL OF CHEMISTRY. 



Volume XXII. 



BOSTON, MARCH, 1888. 



NOHBER 3. 



CONTENTS. 



Familiar Science. — How to Make a Sun-Dial 33 



An Experiment upon Magnetic Induction 33 



On the Study of Nature 34 



Natural Hiatory Notes 35 



Botany as a Ilecreation 35 



Government Meddlesomeness 36 



Interesting Discoveries at Pompeii 36 



Scientific Brevities 36 



Pbacticai. Chemistry and the Arts. — A Rotary Ther- 

 mometer 37 



A Simple Method of Illustrating Chemical Equivalents ... 37 



A Gelatine Battery 37 



Modern Explosives 37 



The Flight of Birds 38 



Levying Duty on Electricity 38 



Laboratory Notes 38 



Industrial Memoranda 38 



Home, Farm, and Garden. — Household Hints 39 



A Device for Measuring Blossoms 39 



Scientific Facts for Housekeepers 39 



Color In Plants 40 



Gleanings 40 



Editorial- — Prehistoric Monuments in the Pyrenees 41 



Berlin Letter 42 



Astronomical Phenomena for March, 1888 . .•. 43 



Meteorology for January, 1888 43 



Do Horses have Horse Sense ? 44 



The Recent Western Blizzard 44 



Literary Notes 44 



Medicine and Pharmact. — The Healing Art 45 



Monthly Summary of Medical Progress 46 



The Hygiene of Phthisis 48 



The Doctor's Wife 47 



On Measuring Liquids by Drops 47 



A Chinese Medicine Advertisement 48 



Medical Memoranda 48 



Humors 48 



PoBLisHERS' Column 48 



f amtltac Sittenct. 



HOW TO MAKE A SUN-DIAL. 

 A SIMPLE sun-dial can be easily made out 

 of wood or pasteboard, which maj- be placed 

 out of doors in any convenient place, or upon 

 the window-sill of a room with a southern 

 exposure, and will indicate the solar time as 

 accurately as the more expensive stone or 

 metal dials. The method of drawing the hour- 

 lines is as follows : Take a piece of wood or 

 pasteboard of any convenient size, and draw 

 upon it the line AB, which will be the noon 

 or Xll-o'clock line upon the dial. Then 

 make the angle BAE, which must equal the 

 latitude of the locality. At Boston this 

 would be about 42^°. From any point upon 

 AB, as S, draw SE perpendicular to AR; 

 and at E draw OEH perpendicular to AB. 

 Measure upon AB, EC equal to ES, and from 

 the point (7 as a centre draw the quarter-circle 

 EF, which is to be divided into arcs of 1.5°, 

 commencing at E. Through these divisions 

 draw the lines Ca, Cb, Cc, Cd, etc. ; and then 

 from A draw the lines passing through a, b, c, 

 d, etc., which will be the hour-lines, where the 

 shadow will fall. Lay off on GE the points 

 Ea', Eb', Ec', Ed', equal to Ea, Eb, Ec, Ed, 

 and draw the hour-lines from A through them 

 in the same way. It should be noted that the 

 Vl-o'clock line is a perpendicular to AB drawn 

 through A. The point to which the Vll-o'clock 



line is drawn is not shown in the diagram, for 

 lack of room ; but it is found in the same 

 manner as the others, — by extending the line 

 EH, and extending the lower dotted line 

 from C till A intersects. The hour-lines 

 before six in the morning and after six in the 

 afternoon are a continuation of the lines on 

 the opposite side of A. Thus, the V hour- 

 line in the morning is found by extending 

 the V hour-line for the afternoon through 

 the point A. The style which is to cast the 

 shadow is of the shape of the triangle EAS, 



with the side AS, which casts the shadow, 

 prolonged to any convenient length. It must 

 be placed upright on the dial in the direction 

 of the line AE. 



To set the dial in position, it is only neces- 

 sary to place it at noon-time so that the shad- 

 ow of the style falls along the XII line. It 

 should also be remembered that the solar, and 

 clock or mean time rarely agree, and the 

 proper correction must be found. This is 

 given for every day- in the year in many 

 almanacs. Thus, if the sun is found to be 

 ten minutes fast, the shadow must fall upon the 

 noon-mark at ten minutes of twelve by the 

 clock. There is also a constant correction to 

 be made, owing to the use of " standard time " 

 in different sections of the country. At 

 Boston this amounts to about fifteen minutes, 

 which must be added to the clock time without 

 regard to the difference between the sun and 

 the clock. Thus, when the clocks at Boston 

 mark a quarter to twslve, the sun is on the 

 meridian, and it is rS.lly twelve o'clock. In 

 the longitude of New York and Philadelphia 

 this may be disregarded, and only the correc- 

 tion as given in the almanac used. 



We can only saj', in conclusion, that these 

 directions are much simpler than they appear 

 at first sight, and that anj' one possessed of 

 an elementaiy knowledge of geometry, and a 

 pair of drawing compasses and a protractor, 

 will find no difficulty in making for himself a 

 useful and entertaining piece of horological 

 apparatus. 



AN EXPERIMENT UPON MAGNETIC 

 INDUCTION. 



If a piece of metal is rapidly moved before 

 the poles of a magnet, currents of electricity 

 are induced in it, and, by their reaction upon 

 the magnet, tend to stop its motion. This 

 phenomenon was first observed by Arago in 

 1824, and was more fuUj^ demonstrated and 

 explained by Faraday. He suspended a 

 cube of copper to a twisted thread which 

 was placed between the poles of a powerful 

 electro-magnet. Left to itself, it began to 

 spin round with great velocitj', but stopped 

 the moment the electro-magnet was excited by 

 a current of electricity. In a similar way, if 

 a knife-blade is moved backwards- and for- 

 wards between the poles of a strong magnet, 

 great resistance is felt, "as if the knife was 

 cutting through cheese ; " and it may even be- 

 come heated the same as by actual friction. 

 This principle is practically made use of in the 

 construction of delicate galvanometers, where 

 the oscillations of the magnetic needle are 

 controlled and "dampened " by the proximity 

 of a mass of copper. 



The illustration shows a very simple and 

 effective way of showing this effect of induc- 

 tion. A top is constructed out of a piece of tin 

 or sheet-iron. When it is at rest, the metal 

 will be attracted by a magnet in the usual 

 way ; but if rapidly rotated, and the magnet 

 brought over it, the induced currents will 



cause it to be repelled by the magnet. The 

 energy of this repulsion will be proportional 

 to the speed of rotation ; and as this dimin- 

 ishes, the repulsion becomes less and less, and 

 at a certain point the attractive influence is 

 again exerted. If the poles of the magnet 

 are held to the edge of the revolving disk, 



— that is, parallel to the plane of rotation, 



— the induced currents will neutralize each 

 other, and no effect will be observed. 



This apparent paradox of the attraction and 

 repulsion by a magnet of the same piece of 



