M. Blundevile his exercises containing sixe treatises, the titles wherof are set down in the next printed page: which treatises are verie necessarie to be read and learned of all yoong gentlemen that haue not bene exercised in such disciplines, and yet are desirous to haue knowledge as well in cosmographie, astronomie, and geographie, as also in the arte of navigation ... To the furtherance of which arte of navigation, the said M. Blundevile speciallie wrote the said treatises and of meere good will doth dedicate the same to all the young gentlemen of this realme.

The double rule called in Latine regula duplex. Cap. 11.

WHereto serues this rule and what order is to be obserued therein?

This rule serueth to vnfold two questions wrapt in one, as thus. If I pay 4. d. for the carriage of 20. l. waight 30. miles, what shal I pay for the carriage of 50. l. waight 60. miles, here of this and such like demaunds, you must make 2. sundrie questions, and the fourth Somme of the first question being found, shall be the second or middle num∣ber of the second question: wherefore frame your first question thus, if 20. l. cost 4. d what shall 50. l cost, and shall find that it will cost you 10. d. then say if 30. miles cost 10. d. what shall 60. miles cost and you shall finde that it will cost 20. d. And note that each of these 2. questions is to bee wrought by the common rule of 3. that is to say by Multiplying the second into the third, and by di∣uiding the product thereof by the first, and the fourth found num∣ber

of the first question must be the second, or middle number of the second question, as in the former example, you sée that 10. d. which was the fourth found number, is here the middle number of the second question.

Another example.

If 25. l. doe gaine me 8. l. in 4. yeares, how much shall a 100. l. win one in 10. yeares, both these questions are also to bee wrought by the common rule of 3. Wherefore set downe the first question thus, if 25. l. yéeldeth 8. l. what shall 100. l. yéelde, and you shall finde 32. l. then say 4. yeares yéeldeth 32. l. what shall 10. yeares yéelde, and you shall find 80. l. But note that these dou∣ble questions, may be put in such sort as you must worke y^e first or second question, sometimes by the rule reuerse. As in this questi∣on here following, if 6. l. win 8. Crownes in 10. yeares, in how many yeares shall 3. l. win 12. Crownes, here frame your first question thus, if 6. l. require 10. yeares how many yeares shall 3. l. require: And in working this question by the rule Reuerse, you shall finde 20. yeares, then for the second question say thus, if 8. Crownes require 20. yeares, how many yeares shal 12. Crownes require: Here if you worke by the common rule of 3. you shall finde 30 yeares.

Another example.

If 7. horses doe eate 12. bushels of Dates in 20. daies, how many bushels shall 14. horses eate in 15. daies, here frame your first question thus, if 7. horses doe eate 12. bushels, what will 14 horses eate, and in working by the common rule of 3. you shall finde in the quotient 24. bushels, then frame your question thus, if 20. daies require 24. bushels, what will 15. daies require, here in working by the common rule of 3. you shall finde in the quoti∣ent 18. bushels.

Another example.

If ten reapers reape 15. Acres in 7. dayes, in how many daies shall 16. reapers reape 20. Acres: Here frame your first question thus, if 10. reapers require 7. daies, how many daies shall 16. reapers require, which question must be wrought by the rule Re∣uerse, and so you shall finde 4. daies and ⅜. of a day which is 9.

houres, then say, if 15. Acres require 4. daies and ⅜. of a day, how many daies shall 20. Acres require, and in working this second question by the common rule of 3. belonging of Fractions as is before taught, you shall finde that 20. Acres wil require 5. daies and 10/12. or ⅚. of a day which is 20 houres. But it were more easie in this second question to reduce the daies into houres by Multi∣plying the 4. daies by 24. houres, the product whereof wil be 96. houres, whereunto if you adde the od 9. houres it will make in all 105. houres, which being Multiplyed by the third number of this second question, which is 20. the product shall be 2100. houres, which diuided by the first number of the said question which is 15. you shall finde in the quotient 140. houres, which if you diuide a∣gaine by 24. you shall finde in the quotient 5. daies, and the re∣mainder to be 20. houres, which agréeth in all pointes with the first manner of working by Fractions, and is the easier way of the two.

How shall I know hauing to worke by this Double rule, when to vse the rule reuerse?

By considering whether the third number requireth more or lesse of time, or of any other measure or quantitie, as in the for∣mer example, of Bayes for lining, the more breadth it had, the lesse did serue for lining. Againe in the example of the gaine by yeares, of 6. l. and 3. l. you did sée that 3. l. require more yeares then 6. l. and therefore that first question was wrought by the rule reuerse: Also in this last example of y^e reapers, the more reapers, the lesse time they require, & therefore that question was wrought by the rule reuerse.