17: 



SCIENTIFIC NEWS. 



[Aug. 24, 1888. 



line of the equinoctial dial, it will fall on the position of 

 the XII. line of the wall-dial, and so on for the other 

 hours. A wall-dial cannot show the hours before VI. in 

 the morning, or after VI. in the evening, unless it faces 

 considerably east or west of south. 



If the dial faces otherwise than due south, and is pro- 

 vided with a gnomon whose plane is at right angles to 

 the dial, it will not stand upon the noon line, as in a 

 true south dial, or a horizontal dial, but will be more or 

 less inclined to it. The line on which the style or gnomon 

 stands is called the substylar line. A dial of this 

 kind may be seen on the tower of St. Clement Dane's in 

 the Strand, near the Law Courts. The substylar line is 

 on the XL line in this case. 



It will be observed that if the dial be arranged for 

 latitude 90, that is, for the pole, the point R, Fig. 8, will 

 be identical with the point F. This will be the centre 

 of the dial, and if the hour lines be drawn it will be 

 found that they simply radiate at equal angles of 15 

 degrees. The proof of this the reader will find to be an 

 easy and not uninteresting problem. 



The following method of dividing a dial was exten- 

 sively used by the dial makers, since not only the hours, 

 but the quarters and even minutes, can be set out with 

 little difficulty. The method consists in the use of scales 

 with specially prepared graduations. These scales are 

 often to be found in old sets of instruments, but their 

 use is almost forgotten. 



There are two scales, the one a " scale of latitudes," 

 the other a "scale of hours." The former is graduated 

 as follows : — From the centre A, Fig. 1 2, draw the circle 

 BCDE, regarding BD as poles, and EC as the equator ; 

 mark off F as the latitude of the place, the angle CAF 

 being the angle of latitude. Draw FG parallel to EC, 

 and from C draw a straight line CGH through G, cutting 

 the circle at H. Join BE. From the centre E, at the 

 distance EH, draw an arc of a circle, cutting EB at K. 

 The point K represents the latitude of the place on the 

 scale of latitudes EB. The scales are usually marked 

 with every degree from o° to 60?. 



The scale of hours is of simpler construction. Divide 

 the arc DE into six equal parts for the four lines, and 

 sub-divide the hours into quarters or minutes. Join ED, 

 and draw the line LM parallel to ED, touching the 

 circle. From the centre A draw lines through the divi- 

 sions on the circle, cutting the line LM. LM will be a 

 scale of hours. 



The scales are used thus : — Draw the meridian line 

 AB, Fig. 13, or if a thick gnomon is to be used draw a 

 double line. Draw the VI. line at right angles, and from 

 the centres of the dial set off on both sides the distance 

 EK on the scale of latitudes, along the VI. line. Then 

 take the scale of hours, and, placing one end at the point 

 thus found, let the other end fall on the meridian line. 

 Straight lines drawn from the centre through the divi- 

 sions on the scale of hours, will give the hour lines and 

 other divisions on the dial. 



On the circumference of a circle of c| inches diameter, 

 a division representing five minutes of time is about one 

 sixteenth of an inch long. 



An ordinary sun-dial will only show the correct time 

 on or about the 15th of April, 15th of June, 1st of 

 September, and on Christmas Day. On other days a 

 correction must be made by adding or subtracting from 

 the time as shown on the dial. This correction is called 

 " the equation of time," and varies slightly from year to 

 year, the second year after leap year being about the 



mean ; this variation, however, amounts only to a few 

 seconds, and therefore does not concern us here. The 

 following table should be provided in some form or 

 other with every dial. The seconds have been omitted, 

 and the days are given on which A stands for "add," 

 and S stands for " subtract." For example, on the 1st 

 of January, when the sun-dial shows 10 o'clock, the real 

 time is four minutes past ten. Again, on the 5th of 

 December, we should have to substract nine minutes 

 from the time as shown on a dial. 

 Jan. 



Feb. 



1— A 



4 



Apr. 



25— S 



2 



Sept 



30— S 



10 



6-A 



6 



May 



15-s 



4 



Oct. 



7-S 



12 



11— A 



8 



June 



5-S 



2 



II 



15-S 



14 



15— A 



10 



M 



14— 







II 



28— S 



16 



23— A 



12 





25 — A 



2 



Nov. 



9-S 



16 



1— A 



14 



July 



5-A 



4 



11 



21— S 



14 



22 — A 



14 



n 



23— A 



6 



11 



26— S 



12 



4-A 



12 



Aug. 



16— A 



4 



Dec. 



3-S 



10 



12 — A 



10 



tj 



25 — A 



2 



II 



8— S 



8 



20— A 



8 



Sept. 



1 — 







tt 



12— S 



6 



25 — A 



6 



n 



7-S 



2 



II 



16— S 



4 



1— A 



4 



„ 



I3-S 



4 



It 



20 — S 



2 



8— A 



2 



n 



18— S 



6 



?l 



25— 







15— 







n 



24— s 



8 



11 



28— A 



2 



Mar. 



Apr. 



This table may be represented by the diagram Fig. 14, 

 which explains itself. It may be arranged in other ways, 

 and there are a number of different modifications of the 

 ordinary sun-dials by which, if the time of year be known 

 to within a day or two, the true time can be read off at 

 a glance. It should be noted that if it is desired that 

 Greenwich time be shown, a constant correction of four 

 minutes per degree of longitude must be made. This is 

 to be subtracted if the place be to the west of Green- 

 wich, and added if it be to the east. 



THE SHIPMAN AUTOMATIC 

 ENGINE. 



'T'HE chief objections to the more general use of small 

 steam yachts and launches are the difficulty in 

 carrying fuel to run them when coal is used, and the 

 cumbersomeness of the engines, which require a trained 

 engineer to keep all in order. These difficulties have 

 been largely overcome by the introduction of the Ship- 

 man automatic steam-engine, of which we give an 

 illustration. The fuel used is petroleum, and the boilers 

 are made of wrought-iron and steel, tested up to 350 

 and 400 pounds hydrostatic pressure ; it is said that 

 they are absolutely safe against either fire or explosion. 

 Steam and oil are discharged together in a very fine 

 spray through an atomizer into the fire-box, the supply 

 of oil being automatically controlled by the steam 

 pressure in the boiler operating upon a diaphragm, 

 which may be set at any desired limit, so that when the 

 pressure reaches this limit, the supply of oil is entirely 

 cut off until the pressure drops again. The oil tank is 

 placed at any convenient distance from the fire-box, so 

 that there should be no more danger in using 

 petroleum, in the way it is supplied by this system, 

 than in ordinary lamps. The water supply is 

 also automatic, the feed water pump being in 

 operation, whenever the engine is in motion, and 

 a ball-float connected to the cut-off valve in the 

 pump regulating the supply. The combustion is as per- 

 fect, and ordinarily as free from smoke, as in a well- 

 trimmed lamp. Until the steam pressure is obtained in 

 the boiler, an air pump is worked by hand to feed the 



