The mariners magazine, or, Sturmy's mathematical and practical arts containing the description and use of the scale of scales, it being a mathematical ruler, that resolves most mathematical conclusions, and likewise the making and use of the crostaff, quadrant, and the quadrat, nocturnals, and other most useful instruments for all artists and navigators : the art of navigation, resolved geometrically, instrumentally, and by calculation, and by that late excellent invention of logarithms, in the three principal kinds of sailing : with new tables of the longitude and latitude of the most eminent places ... : together with a discourse of the practick part of navigation ..., a new way of surveying land ..., the art of gauging all sorts of vessels ..., the art of dialling by a gnomical scale ... : whereunto is annexed, an abridgment of the penalties and forfeitures, by acts of parliaments appointed, relating to the customs and navigation : also a compendium of fortification, both geometrically and instrumentally / by Capt. Samuel Sturmy.

CHAP. XII. How to make the Cross-Staff.

THe Mariner's Cross-Staff is that which by the Astronomers is called Radius Astronomicus, by which we observe the Celestial Lights above the Horizon. The Mathematicians have invented many kinds of Instruments, whereof the Cross-Staff and Quadrant are the most useful above all the rest. At Sea it is not every Mans Work to make and mark a Cross-Staff, and other Instruments, for want of Practice needful thereunto; yet notwithstanding it is fit and necessary that a Master, his Mates, and Pilot, who are to have the Use of it, should at least know when it is well ••ade.

For to mark well a Cross-Staff, you shall make a plain flat Board of good dry

Wood, fifteen or sixteen Inches broad, and about four Foot or three Foot long: Paste it well with good Paper; draw along the one Side a Right Line, as in the next fol∣lowing Figure CAD; out of the Line C draw a Square Line upon AC, as CB, and upon the Center C draw the Arch AEB, being a Quadrant or fourth part of a Circle; divide that into two parts; the one half thereof, as AE, divide into 90 Equal Parts or Degrees, thus; first into three Parts, and each of the same again into three; these Parts each into two, and each of the last Parts into five: so the Arch AE shall then be divided into 90 Parts. Then take a right Ruler, lay the one end on the Point or Center C, the other upon each Point of the foresaid several Divisions, and draw small Lines out of C, through each of the foresaid Points or Degrees of the Quadrant, so long as they can stand upon the Board, as you may see it plain in the Figure. Then take with a pair of Compasses, just the half length of the Cross that you would mark the Staff after; prick it from the Point C towards B; as by Exam∣ple, from C to F, and from D to G; joyn these two Points with a Line to one ano∣ther; and even into such Parts as that Line is cut, and divided by the aforesaid Lines coming out of the Center of the Quadrant, must your Staff be marked, whether the Cross be long or short, as appeareth by the Lines HI and KL, which are drawn for Crosses: the half thereof is so long as CH, or CK, or CF. If the aforesaid Quadrant, for want of good Skill or Practice, be not well divided, or Lines not well drawn, the Staves being marked thereafter will also be faulty. Therefore they may be marked more exactly by Points equally divided, in manner as followeth.

[illustration] geometrical diagram

A Table for the Division of the Cross-Staff.

D.	Parts	D.	Parts.	D.	Parts.
1	176	31	7675	61	28667
2	355	32	8040	62	30108
3	538	33	8418	63	31653
4	724	34	8807	64	33315
5	913	35	9210	65	35107
6	1106	36	9626	66	37046
7	1303	37	10057	67	39152
8	1504	38	10503	68	41445
9	1708	39	10965	69	43955
10	1918	40	11444	70	46713
11	2131	41	11943	71	49758
12	2349	42	12460	72	53197
13	2572	43	12998	73	56912
14	2799	44	13558	74	61154
15	3032	45	14142	75	65958
16	3270	46	14751	76	71445
17	3514	47	15386	77	77769
18	3764	48	16051	78	85144
19	4019	49	16746	79	95854
20	4281	50	17475	80	104301
21	4550	51	18239	81	117062
22	4826	52	19042	82	133007
23	5108	53	19887	83	153499
24	5399	54	20777	84	180811
25	5697	55	21716	85	219038
26	6003	56	22708	86	276332
27	6318	57	23759	87	371885
28	6643	58	24874	88	561810
29	6976	59	26059	89	1135891
30	7320	60	27321	90	Endless.

Prepare you a Staff, draw thereon a Right Line so long as your Staff, and take with a sharp pair of Compasses the half Length of the Cross after which you desire to mark your Staff: prick it so often along the aforesaid Line, as it can stand upon the same. Divide each of the Lengths of the half Cross into 1000 Equal Parts. Then prick upon the Staff you will mark from the Center-end, just half the Length of the Cross; and mark there a small thwart Stroke. Off from thence prick for each Degree so many of the same Parts as the Cross is divided in his half Length, like as is marked in the Table here annexed for every Degree. For the first Degree you shall mark off from 90 the aforesaid thwart Stroke 176, for the fourth Degree 724, for the 10 Degree 1918 of those Parts, and so of the rest. If you cannot divide the half Cross, by reason he is so little, into 1000, divide him into 100, and leave out the two last Figures, and that shall satisfie your desire: For 30 De∣grees take 73, and for 40 Degrees 114, and for 10 Degrees 19 Parts, and so of the rest.