A Treatise on Mathematical Instruments, Including Most of the Instruments Employed in Drawing, for Assisting the Vision, in Surveying and Levelling, in Practical Astronomy, and for Measuring the Angles of Crystals: In which Their Construction, and the Methods of Testing, Adjusting, and Using Them, are Concisely Explained

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J. Weale, 1859 - 170 páginas
 

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Página 148 - ... half by turning the small screw at one end of the level. Repeat the operation till the bubble retains the same position in both positions of the level, and the axis will be horizontal. To adjust the Line of Collimation in Azimuth. — Direct the telescope to some distant, small, and well-defined object, and bisect it by one extremity of the middle vertical wire, giving the telescope the azimuthal motion necessary for this purpose by turning the screw s. By elevating or depressing the telescope,...
Página 143 - ... for that being constant, the result would in some measure be vitiated thereby. " When both the reflected and direct images require to be darkened, as is the case when the sun's diameter is measured, and when his altitude is taken with an artificial horizon, the attached dark glasses ought not to be used : instead of them, those which apply to the eye-end of the telescope will answer much better ; the former having their errors magnified by the power of the telescope, will, in proportion to this...
Página 101 - ... is out of adjustment, and requires correcting. The end to which the bubble retires must then be noticed, and the bubble made to return one.half the distance by turning the parallel plate screws, and the other half by turning the capstan.headed screw at the end of the bubble. tube. The telescope must now again be reversed, and the operation...
Página 131 - Then press downwards the branches ee, which will cause the points to make punctures in the paper at opposite sides of the circle ; which being afterwards connected, the line will pass through the given angular point, if the instrument was first correctly set. In this manner, at one setting of the instrument, a great number of angles may be laid off from the same point. It is not essential that the centre be over the given point, when applied to...
Página 125 - ... of its run by turning these screws. The bubble will now retain its position while the vernier plate is turned completely round, showing that the internal azimuthal axis about which it turns is truly vertical. The bubbles of the levels on the vernier plate, being now, therefore, brought to the centers of their tubes, will be adjusted to show the •vertically of the internal azimuthal axis.
Página 93 - Ten arrows must be provided with the chain, about 12 inches long, pointed at one end, so as to be easily pressed into the ground, and turned at the other end, so as to form a ring, to serve for a handle. In using the chain marks are first to be set up at the extremities of the line to be measured. Two persons are then required to perform the measurement. The chain leader starts with the ten arrows in his left hand, and one end of the chain in his right, while the follower remains at the starting...
Página 24 - From the center at a draw the line ag for the axis of the gnomon agi, and from g let fall the perpendicular gi upon the horizontal meridian line an, and there will be formed a triangle ag i.
Página 9 - ON SCALES. Scales of equal parts are used for measuring straight lines, and laying down distances, each part answering for one foot, one yard, one chain, &c., as may be convenient, and the plan will be larger or smaller as the scale contains a smaller or a greater number of parts in an inch. Scales of equal parts may be divided into three kinds ; simply divided scales, diagonal scales, and vernier scales.
Página 133 - E., and W. respectively. Add up these several columns, and, if the work is so far correct, the sum of the northings will equal the sum of the southings, and the sum of the eastings will equal the sum of the westings. Then in two additional columns enter the whole quantities of northing and easting, made at the termination of each of the several...
Página 102 - Place the instrument half way between the stakes a and b, and read the staff A, placed on the stake a, and also the staff B, placed on the stake b ; call the two readings, A...

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