A Treatise on the Principal Mathematical Instruments Employed in Surveying ...

Front Cover
T. Bensley, 1834 - 100 pages

From inside the book

Other editions - View all

Common terms and phrases

Popular passages

Page 40 - ... to make the objects appear on the other wire ; if the contact still remains perfect, the axis of the telescope is in proper adjustment ; if not, it must be altered by moving the two screws which fasten, to the up-and-down piece, the collar into which the telescope screws. This adjustment is not very liable to be deranged.
Page 33 - ... triangle, of which the perpendicular is the difference of level. It scarcely appears necessary to give the rule for the calculation, but for the sake of uniformity we shall do so.f Add together the logarithm of the measured distance, and the log. sine of the observed angle; the sum, rejecting 10 from the index, will be the log. of the difference of level, in feet or links, efec., the same as the distance was measured in.
Page 24 - The first adjustment is that of the line of collimation ; that is, to make the intersection of the cross wires coincide with the axis of the cylindrical rings on which the telescope turns : it is known to be correct, when...
Page 44 - Prepare the instrument for observation by screwing the telescope into its place, adjusting the drawer to focus, and the wires parallel to the plane, exactly as you do with a sextant : also set the index forwards to the rough distance of the sun and moon, or moon and star ; and, holding the circle by the short handle, direct the telescope to the fainter object, and make the contact in the usual way. Now read off the degree, minute, and second, by that branch of the index to which the tangent screw...
Page 44 - ... but even here, you have a mean of that altitude, and this error, taken on three different sextants. Both at sea and land, where the observer is stationary, the meridian altitude should be observed forwards one day, and backwards the next, and so on alternately from day to day; the mean of latitudes, deduced severally from such observations, will be the true latitude ; but in these there should be no application of index-error, for that being constant, the result would in some measure be vitiated...
Page 45 - But what is of still more consequence, the error of the center is perfectly corrected by reading the three branches of the index ; while this property combined with that of observing both ways, probably reduces the errors of dividing to one-sixth part of their simple value. Moreover, angles may be measured as far as one hundred and fifty degrees, consequently the...
Page 13 - ... any deviation in it is easily rectified, by releasing the screws by which it is held, and tightening them again after having made the adjustment ; or, what is perhaps better, note the quantity of deviation as an index error, and apply it, plus or minus, to each vertical angle observed. This deviation is best determined by repeating the observation of an altitude or depression in the reversed positions, both of the telescope and the vernier plate: the two readings will have equal and opposite...
Page 39 - The amount of the index error may be found in the following manner: clamp the index at about 30 minutes to the left of zero, and looking towards the sun, the two images will appear either nearly in contact or overlapping each other ; then perfect the contact, by moving the tangent-screw, and call the minutes and seconds denoted by the vernier, the reading on the arc. Next place the index about the same quantity to the right of zero, or on the arc of excess, and make the contact of the two images...
Page 46 - This is performed by a capstan screw, at the lower end of the frame of that glass ; and is known to be right when by a sweep of the index the reflected image of any object will pass exactly over, or cover the image of that object seen directly. The third adjustment is for making the line of collimation parallel to the plane of the circle. This is performed by two small screws, which also fasten the collar into which the telescope screws to the upright stem on which it is mounted ; this is known to...
Page 30 - AC 2AC nearly ; that is, the difference between the true and apparent level is equal to the square of the distance between the places, divided by the diameter of the earth ; and consequently it is always proportional to the square of the distance.

Bibliographic information