55 



NATURE 



\May 8, 1879 



circumfejrence from i to 31. Two openings on a hori- 

 zontal diameter allow drums to show the month and day 

 of the week respectively, and a central hand points out 

 the day of the month. A cam, formed lilce the snail of 

 an English striking-clock, but without the steps, is caused 

 to rotate once in twenty-four hours by the clock move- 

 ment, so that a pendant, resting on it, is raised through a 

 space of about i inch in that period and allowed to fall, the 

 weight being supplemented by the tension of a spiral spring; 

 this is the sole connection between the calendar and clock. 

 During the ascent of the pendant a detent passes over one 

 tooth of a wheel fixed to the week-day drum, which is thus 

 carried round through a corresponding interval when the 

 release occurs. At the same time a precisely similar 

 action, performed on a wheel fixed to the axis that carries 

 the hand, causes it to advance one figure. 



Just as the cam driven by the clock accomplishes the 

 change from day to day, so a second cam on the central 

 axis of the calendar alters the month ; the detent, on 

 being released, carries forward one tooth of a 12-toothed 

 wheel. It remains to explain the device for allotting the 

 requisite number of days to each month and correcting 



I-day Parlour Calendar, No. 4. Height 25 inches. Spring-Strike. 

 8-inch Dials. 



for leap year. The axis of the month drum carries an 

 irregular shaped cam, which may be conceived to be 

 divided radially into twelve parts. Those arcs of the 

 circumference that correspond to 31-day months arc left 

 untouched ; 30-day months have their arcs filed away to 

 the corresponding chord ; and for February a depression 

 is made equal to three times that of other months such as 

 April. A hght spring holds a bent arm against this cam, 

 the arm being so placed that at the end of each short 

 month it can ride on a metallic arc carried round with 

 the hand ; the acting length of this arc corresponds to 

 one or three teeth of the dial-wheel if the 30th or 28th is 

 the last day, and the arm entirely escapes it when thirty- 

 one days are to be indicated. Whenever it is thus held 

 out of its natural position, the arm prevents the check- 

 spring that limits the movement of the dial-wheel from 

 falling into its place, and the detent is thus enabled to 

 advance the hand through two or four spaces instead of 

 the usual one. An additional day is given in leap-year 

 by a simple application of the well-known sun and planet 

 wheel of Watt. The central fixed wheel is coaxial with 

 the month-drum and has sixteen teeth ; the planet-wheel, 

 pivoted on the cam, has twenty teeth, and carries a sector 



of such a radius that, when superposed on the February 

 depression, it diminishes the fall of the arm so that it 

 rides on an arc corresponding to two teeth instead of 

 three. It will be seen that the above numbers of teeth 

 are so chosen that the wheel carrying this sector is only 

 brought into an identical position once in every four 

 (annual) rotations of the month-drum ; the necessary 

 correction is therefore effected. 



SPIRAL SLIDE RULE^ 



THE method of multiplying and dividing by means of 

 a rule was first introduced by Gunter about the year 

 1606 by the construction of a scale of two equal parts 

 divided logarithmically, the readings being taken off with 

 a pair of compasses. Oughtred about 1630 invented the 

 rule composed of two similar logarithmic scales sliding in 

 contact, but the difficulty of estimating the reading 

 between two graduations then first became important. 

 It is easy to see that it requires but little practice to place 

 a graduation in one scale opposite to a position obtained 

 by estimate between two graduations in the other scale, 

 but it becomes a much more tiresome and uncertain pro- 

 cess when both of the readings required to be placed i» 

 juxtaposition fall between two graduations on their re- 

 spective scales. With practice, however, this operation 

 can be effected with considerable accuracy provided the 

 graduations are not too close together ; hence to enable 

 the calculations to be performed with a sufficient degree- 

 of approximation there has always been a desire to 

 increase the scale and consequently 

 the total length of the instrument. To 

 attain this object and at the same time 

 preserve the portable size of the in- 

 strument Prof. Everett designed his 

 slide rule, but the range of this is now 

 far surpassed by the invention by Prof. 

 Fuller of the spiral slide rule. 



The instrument can be readily under- 

 stood from the accompanying figure. 



</ is a cylinder that can be moved up 

 and down or turned round on the 

 cylinder ff, attached to and held by 

 the handle e. Upon d is wound in a 

 spiral a single logarithmic scale. Two 

 other indices, c and a, whose distance 

 apart is equal to the axial length of 

 the spiral, are attached to the cylinder 

 g, which slides in / and thus enables 

 the operator to place them in any re- 

 quired position relative to d. o and p 

 are two stops which when placed in 

 contact bring the index b to the com- 

 mencement of the scale, m and n are 

 two scales, one attached to the movable 

 indices and the other to the cylinder d. 



By the spiral arrangement the length 

 of the scale can be made very great, 

 and as only one scale is required the 

 effective length is double that of an 

 ordinary straight rule. The scale is 

 made 500 inches, or 41 feet 8 inches 

 long, and the instrument is thus equi- 

 valent to a straight rule 83 feet 4 inches 

 long or a circular rule 13 feet 3 inches 

 in diameter. The first three digits of 

 a number .are printed on the rule 

 throughout the scale, much increasing 

 the facility of reading off. The method 

 of using the different indices will be best understood by 

 examples. For multiplication — bring 100 to the fixed 



/ 



;^^^ 



7» pT 



index b and place the movable index to the multiplicand, 



» By George Fuller, M.InstC.E., Professor of Engineering, Queen's Uni- 

 versity, Ireland. 



