Transit Instrument
A telescope mounted at right angles to a horizontal axis. Used in connection with a clock or watch for obtaining the time of transit of a heavenly body over the meridian of a place. To watchmakers who make any pretense of a knowledge of their business, nothing can be more desirable or useful. A vague impression exists among them, that the transit instrument, used for this purpose, is so closely allied to the scientific, as to be serviceable only in the hands of the professional astronomer. This fact, taken in connection with the high cost of these instruments hitherto, fully accounts for the reason of their unfrequent employment.
These two causes, preventing the more general use of the transit, no longer remain, whatever may have been their force in the past. By an improvement in the mode of mounting, a ready means is furnished for setting up and placing the instrument in the meridian. Furnished with each transit are full printed instructions, describing each part in detail, the method of setting up, and taking observations, the whole of which is so plain and simple that every purchaser is surprised and gratified to find what was supposed to be difficult so easy to perform.
The principle involved in the use of the transit instrument is, that when in proper position, the center vertical line in the field of view of the telescope, shall be exactly on and represent the meridian, or true north and south line, of the place of observation. Hence, from the observed time of any heavenly body crossing the center or meridian line, may be determined the error of the time-piece used. The passage of any heavenly body across the meridian line is called its transit.
The stand on which the instrument rests, consists of two circular plates of metal, the lower of which, A, is to be screwed to a foundation of wood or stone; screw holes being provided for that purpose. The upper plate, B, turns upon the lower by means of a pivot, and the two are clamped together by a clamping screw. In the upper surface of the plate B, are sockets for receiving the three points on which the frame of the instrument rests. When the clamping screw is loosened, the entire instrument may be easily rotated horizontally upon the lower plate A, and when the champing screw is tightened, the whole is rigidly held in position.
The telescope is ten inches long and fitted with an axis, the two pivots of which rest in the top of the frame F. At X is a sliding tube, which adjusts the focus of the object glass. The telescope is furnished with a diagonal eye-piece, E, which is made movable in a sliding tube at Y, to adjust the focus of the eye-piece. Small screens of colored glass are provided to protect the eye from the effects of the rays of the sun. On the axis of the telescope is attached a declination circle, D, used only for facilitating the finding of stars, when on or near the meridian, but not used in taking observations of the sun. Upon the pivots of the telescope is placed a riding level, for the purpose of leveling the axis of the telescope by means of the leveling screw L.
One of the pivots of the axis rests in a block made moveable by the horizontal adjusting screw.
The instrument must command an unobstructed view of the north star, and the sun at noon. Usually, the coping of a brick or stone building, easy of access, will be found the most convenient, but in the absence of this, a brick or stone pier, from sixteen to twenty-four inches square, built from four feet below to four feet above the ground, would be excellent, or a post six inches or more in thickness, firmly set in the ground would answer a very good purpose. A costly foundation offers no practical advantage and its greater expense need not be incurred. At the place chosen secure very firmly, a piece of plank two or three inches thick, of any suitable size, which should be well leveled, and thoroughly painted, top and bottom, to protect it against the action of the weather, and upon this plank screw the lower bed plate of the instrument. The instrument may be covered with a tight box, constructed to turn water and exclude dust, or, if removed after each observation, a tight cover should be made for the rotating stand, which must be left in its position.
The instrument is placed in position, or on the meridian line, by setting the center line in the field of view of the telescope, on the north star, at the time of night the said star is on the meridian, which time is calculated for any desired place and furnished in tabular form with each transit. Perfectly correct time for this operation, which is very simple, is not essential, but only as near as is in common use.
The instrument must now remain unmoved until a range mark has been obtained as follows:
On the day succeeding the setting of the center line on the north star, turn the telescope to the south, and notice some small object of a permanent character, which the center line bisects. This object is called the range, and may be at any distance from twenty feet to half a mile, but two or three hundred yards will be found best. If no convenient object can be selected, a range may be made by painting a small round spot on a permanent surface, at a point where the center line will bisect it. Before taking an observation the center line must always be set exactly on the range with the screw H, in order to secure an accurate adjustment in the meridian. The range may be either north or south, but the latter is the more convenient. The purpose of the range mark is to furnish a means of keeping the instrument in the meridian without being obliged to re-set the center line on the north star.
METHOD OF TAKING AN OBSERVATION OF THE SUN.
Set the center line on the range, if off the same, and carefully level the axis, a few minutes before the sun comes in the field of view, which will be about four minutes before reaching the center line. As the telescope is an inverting one, the sun will appear to come in from the right hand side and pass across the field of view.
When the first, or advancing edge of the sun intersects the center line, the time must be noted by the watch, as in example. When the sun has passed entirely across the center line, which takes about two and a quarter minutes, note the time when the last edge intersects the center line, as in the same example.
Time of contact of first edge of sun with vertical line--- 11 43 14.5 Time of contact of last edge of sun with vertical line---- 11 45 30. Divide by 2----------------------------------------------- 2)23 28 44.5 ---------------------------------------------------------- 11 44 22.2 Add correction (from table supplied with instrument)------ 16 12.5 ---------------------------------------------------------- 12 0 34.7 Subtract 12 hours----------------------------------------- 12 Watch fast 34 seconds, 7 tenths--------------------------- 0 34.7 After adding the above correction, the difference between the result and 12 hours will be the amount the watch was fast or slow; fast, if more than 12 hours, and slow if less.