What is the best way to combine Sentences 14 and 15? I thought about napkins for a long time I could not come up with something magnificent. I thought about napkins for a long time, and I could not come up with something magnificent. I thought about napkins for a long time, so I could not come up with something magnificent. I thought about napkins for a long time, but I could not come up with something magnificent.

I thought about napkins for a long time, but I could not come up with something magnificent.

Explanation:

1. I thought about napkins for a long time I could not come up with something magnificent Here there is no conjunction  to  enable the two sentence make a contrasting or co-joining  meaning

2.  I thought about napkins for a long time, and I could not come up with something magnificent.

The conjunction AND is not the best way to combine the two sentences. "and" in sentences is used to connect two actions, with one following the other.

3. I thought about napkins for a long time, so I could not come up with something magnificent

So is used as a  subordinating conjunction used to express reason for an action and therefore cannot be applied here.

4. I thought about napkins for a long time, but I could not come up with something magnificent.

The best way to combine the sentences is the use of but which is used here to express a contrast( difference ) in an action

no its not a run on sentence however i think you meant wet suits not sours also a run on sentence would be like surfers surf all winter long in southern california by using wet sours to keep them warm and the live their all year long  

the illustration   that would give a reader a clear understanding of the text is a picture of men constructing the frame of a building.the frame is made of iron bars. the text describes the job of blacksmiths'. these were very organised and they had different names: "the driver" and "the heater". the text describes the way in which these men worked ; they worked iron pieces as if they were playing a sport. in fact, the text wants to show the men's skills. the text also puts some emphasis on the use of '' rivets'' in construction. this kind of nails were not always used in the construction of manhattan.

some examples:

"and with an underhand toss he hurled the smoking rivet straight at the catcher, who caught it in mid-air in his catching can." men's skills

"when the driver was ready for a new " the emphasis on the rivet ( special nail used in construction).

" it against a beam to remove any cinders, and jammed it into the waiting hole. " the emphasis on the rivet.