Page:Popular Science Monthly Volume 88.djvu/461

 Popular Science Monthly

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��For example, you wish to ascertain the latitude of San Francisco, and make your observation on October 23.

1. Ascertain your north-south line. (The sun's shadow will cross it on that date at 11 h. 54 m. 33 s. A. M., Pacific time.)

2. The sun's altitude, when the shadow is north, would be found to be 41°.

3. The declination is 11° S. Adding, we get 52°, the altitude of the celestial equator.

4. Subtract: 90° — 52° equals 38°, the latitude of the place of the observer.

The "analemma" employed is a care- fully worked-out diagram, giving the position of the apparent sun and its declination for every day in the year. It must be remembered that, each year, this will vary slightly, but for all ordinary calculations, the diagram here given will answer every purpose.

The vertical lines represent the number of minutes the apparent sun is slow or fast — as compared with the mean sun. Since the analemma shows how fast or slow the sun is each day, it is obvious that, knowing one's longitude, one can set his watch by the sun, by reference to this diagram ; or, having correct clock time, one can ascertain his longitude.

To ascertain longitude, one must have a true north-south line ; also the correct standard time. Now —

1. Note when the .sun's shadow is due north. Refer to your analemma and see how far the sun is fast or slow.

2. If fast, add the amount to the time by your watch; if slow, subtract. This gives you mean local time.

3. Divide the number of minutes and seconds past or before 12 by 4. This Avill give you the number of degrees and min- utes you are from the standard time meridian. If the right time is before 12, you are east of it; if after, you are west of it.

4. Subtract (or add) the number of degrees you are east (or west) of the standard time meridian, and this gives you your longitude.

To set your watch you must have a correct north-south line and know your longitude.

1. Find the difference between your longitude and that of the standard time meridian by which you wish to set your

��watch — Eastern time. Central time, etc. — as the case may be.

2. Multiply the number of degrees and seconds of the difference by 4. This gives you the number of minutes and seconds your watch is faster or slower than local time. If you are east of the standard meridian, your watch must be set slower than local time ; if west, faster.

3. Observe the position of the sun — whether fast or slow — according to your analemma. If fast, subtract that time from the time obtained in step two ; if slow, add. This gives you the time be- fore or after 12 when the shadow will be north; before 12 if you are east of the standard time meridian; after 12 if you are west.

4. Set your watch at the time indicated by step 3, when the sun's shadow crosses the north-south line.

To strike a north-south line you must know your longitude and have correct time. Steps, 1, 2 and 3 are just the same as before (in the last example). At the moment of making step 3, you know the shadow is north ; then draw the line of the shadow. If out of doors, stakes will indicate this line.

A Vulcanizer for Tire Repairs

SEVERAL new types of vulcaniz- ing devices have re- cently been placed on the market for the motorist who desires to make his own quick tire repair on the road. They all naturally strive to utilize some material or part of the car. Among those of more than passing interest is one which can be used without special instructions. It is nothing more than a clamp, in which the inner tube is held. On the upper half of this clamp is a hollow, to be filled with gasoline. A one-ounce measure goes with the device, and thf ounce of gasoline will burn about seven minutes, which is just enough to effect a complete repair of a puncture.

Another device consists of a plain metal plate which is held by any sort of clamp to the exhaust pipe of the muffler. Putting the inner tube on this metal plate and holding it down on it for about five minutes is sufficient for vulcanizing.

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