docs: write journal entry

Signed-off-by: André Jaenisch <andre.jaenisch@posteo.de>

journal/2021-08-15.md

@@ -21,7 +21,7 @@ I implemented gravity and have first boundary and collision checks in place:
 
 [commit diff 59586dc4684b093e9f68518f114e62100142460b to abe72d67026177be6b028ff21f1b9fd9031cb455][diff]
 
-![screenshot from 2021-08-13][screenshot]
+![screenshot from 2021-08-15][screenshot]
 
 ## Next steps
 
@@ -37,13 +37,18 @@ up in life support systems as well as transport. It could look a bit like in
 ### Codewise
 
 - [ ] See ToDos from [yesterday][yesterday].
-- [ ] Investigate, why box and boundary don't collide.
+- [x] Investigate, why box and boundary don't collide.
 
 ### Previous in line
 
 See journal entry from [yesterday][yesterday].
 
+### Next in line
+
+See journal entry from [tomorrow][tomorrow].
+
 [diff]: https://jaenis.ch/hobbies/coding/repos/ryuno-ki/js13kgames-2021/compare/59586dc4684b093e9f68518f114e62100142460b...abe72d67026177be6b028ff21f1b9fd9031cb455
 [sao]: https://github.com/SecSamDev/HealthBarSAO
 [screenshot]: ./2021-08-15.png
+[tomorrow]: ./2021-08-16.md
 [yesterday]: ./2021-08-14.md

journal/2021-08-16.md

@@ -0,0 +1,49 @@
+# 2021-08-16
+
+I am now *master of the ~galaxy~ gravity*!
+
+Ahem.
+
+I mean, I can now control the gravity via input range so I can control the
+movement of my box.
+
+The precondition was solving the collision detection from yesterday.
+I took the time and [explained the logic][cramer] in an issue against xem's
+repo.
+
+If you have more parts that pique your interest, let me know. I plan to write
+more about geometric algebra as I go along.
+
+## Today's achievement
+
+I implemented gravity and have first boundary and collision checks in place:
+[Play it online][demo].
+
+[commit diff 4f857eeb10b97e5df1383942aa653ef8fe1e417d to f5eca1e2964b0a4625ab2e7acfbd52234ba01814][diff]
+
+![screenshot from 2021-08-16][screenshot]
+
+## Next steps
+
+Sadly, `canvas` (the package) is required by `jsdom` to manipulate a canvas
+element. That would mean a compilation, which is reportedly not possible on
+CentOS (what my hoster uses). I have to rip out JSDom as a consequence.
+
+Zach recommended [linkedom][linkedom].
+I expect this problem to take considerable amount of my time.
+
+### Codewise
+
+- [ ] See ToDos from [yesterday][yesterday].
+- [ ] Replace JSDom with an alternative.
+
+### Previous in line
+
+See journal entry from [yesterday][yesterday].
+
+[cramer]: https://github.com/xem/mini2Dphysics/issues/3
+[demo]: https://ryuno-ki.github.io/js13kgames-2021/
+[diff]: https://jaenis.ch/hobbies/coding/repos/ryuno-ki/js13kgames-2021/compare/4f857eeb10b97e5df1383942aa653ef8fe1e417d...f5eca1e2964b0a4625ab2e7acfbd52234ba01814
+[linkedom]: https://github.com/WebReflection/linkedom
+[screenshot]: ./2021-08-16.png
+[yesterday]: ./2021-08-15.md