Epilogue:
I went out to the patio this morning and noticed a handful of loose marbles in the birdbath. They looked nice, complimenting the sparkly gazing ball... but...
It turned out they had simply fallen off the bowling ball. Oops.
This will be easy to repair, but what I've learned is that I should be more generous with the glue, use the high/strong hold setting, and I think it would be worthwhile trying to scuff up the bottoms of the marbles on a piece of sandpaper before gluing them to the ball.
Also, these marbles are not all the same size, nor are they uniformly circular. They do not pack evenly. Having some of the ball show through is not a problem for me on this project, but it's worth keeping in mind for other patterns and other backgrounds. I've also seen images where the mosaic pieces were set in grout. That will FILL the empty spaces, and depending on the color of the grout, change the look of the surface. Overall, I'm happy with the result. I'll re-glue the marbles that dropped off overnight later when it cools down.
