95ii 



EECBEATIYE SCIENCE. 



resting to those who have leisure to investigate 

 them. We propose to confine our attention, 

 in this article, to a meridian-line ; by which 

 the time may be obtained to within two or 

 three seconds, and which may be of very 

 great use to amateurs in astronomy, etc. 



The meridian-line is a very old method of 

 determining by a fixed mayk, or set of marks, 

 the time of the sun's passing the meridian of 

 the place. There ia a description of one in 

 " Ferguson's Lectures" (1793, 8th edition, p. 

 375). Probably other works also contain 

 notices of similar methods. We are not 

 aware, however, that any one is so useful as 

 the one proposed to be described, which ap- 

 pears to us to combine the chief advantages 

 of all the others with which we are ac- 

 quainted, while it also has some which are 

 original. 



We preface our description by the remark 

 that the meridian-line is not self-checking (so 

 to speak) as the transit-instrument is. Its 

 position must be checked by other means, 

 but when once proved, it may then be re- 

 garded as a fixed line of reference, by which 

 to ascertain the time of the sun's passing 

 the meridian. 



We will suppose two classes of observes, 

 who may be interested in pursuing this sub- 

 ject. The first we will suppose to be able to 

 calculate the error of the clock or watch by 

 taking a set, or sets, of altitudes — single or 

 double' — with the sextant. To do this re- 

 quires some knowledge of trigonometry, and 

 the use of logarithms. This class of persons 

 will, of course, have no difficulty in at once 

 getting the time approximately, in order to 

 fix the line nearly in its position ; and after- 

 wards, by a series of approximations, deter- 

 mine its final place. The other class we will 

 suppose to be ignorant of the trigonometrical 

 modes of obtaining the time. Such persons 

 may, notwithstanding, be fully competent to 

 use the meridian-line when once its position 

 has been carefully determined. The former 

 class can prove at any time how nearly tho 



line is in the plane of the meridian. The 

 latter class must put faith in some one who 

 is competent to ascertain the time for them, 

 and get such an individual to furnish them 

 with the true time in the first instance. They 

 may then tliemselves fix their line once for 

 all, according to the directions liereafter 

 given ; bearing in mind the fact, that when 

 once properly fixed, the line may be regarded, 

 for all rough astronomical purposes, as a final 

 reference, and an independent means of check- 

 ing the clock from day to day, or week to 

 week, etc. 



The mode is as follows : — 



Get a plate of brass or copper of the size 

 say 6 inches by 3, and about l-16th of an inch 

 thick. File a clean hole towards one end of 

 it, say 1 inch long by l-8th broad ; the metal 

 to be fixed so that the hole shall have the 

 long sides vertical. This piece of metal is to 

 be fixed very securely to the upper stone- 

 work outside a window having a southerly 

 aspect — not necessarily opposite the south; 

 my window, where such a line is fixed, for 

 instance, faces about S.S.E. Get the local 

 time approximately, as above referred to, say 

 to about one minute of the truth, in order to 

 ascertain about where to fix a piece of wood, 

 from which a plumb-line is to be suspended, 

 of moderately thick silk or cotton, waiting, 

 of course, till the sun is in the meridian, as 

 nearly as the watch or clock at present can 

 give it. 



Having made a mark on the ceiling, fix a 

 piece of wood firmly to the joists, and on this 

 fasten another piece ; mahogany, or other 

 hardish wood. The latter must be shaped 

 something like the top of the capital letter T, 

 to admit of a wire, copper or brass — with a 

 carefully filed or turned groove in it, to which 

 the plumb-line is to be fixed — being placed 

 through the ears of the T, at right angles to 

 the meridian, and made to move east or west, 

 and "friction-tight," for adjustment. The 

 piece of wood fixed to the joists must be at 

 such a distance from the window as will allow 



