In his biography of Gauss, G. Waldo Dunnington describes the Gauss-Bolyai episode and their correspondence. In particular, he describes the contents of one letter from Gauss to Janos-Bolyai:

In the above mentioned letter Gauss gave as a sample of his own research a proof that in non-euclidean geometry the area of a triangle is proportional to the deviation of the sum of the angles from 180 degrees... In the same letter Gauss urged Johann to busy himself with the corresponding problem for space, namely, to determine the cubic content of the tetrahedron (space bounded by four planes). The papers of Johann contain several processes which can serve as a solution, among them the method which Gauss had in mind and which he indicated in one of his notebooks at the time of sending his letter to Wolfgang on March 6, 1832.

In addition, Stillwell's book "Mathematics and Its History" (p.379) emphasizes that Gauss "did have many of the results of non-euclidean geometry by this time, including the answer to the volume problem he raised to test his young rival (see Gauss (1832 - Cubierung der Tetraeder)". Looking at the same letter, I also saw that Gauss wrote that unlike the case of 2-dimensional content of simplexes (triangles), where the area is proportional to the angular deficit, in the case of 3-dimensional content of tetrahedrons such a simple formula does not exist.

A translation of Gauss's note is:

In the tetrahedron $1234$, whose faces $124$ and $134$ are orthogonal. Its volume is $$\Delta,$$ then it holds that: $$\partial \Delta = -24\cdot\partial341,$$ and if the face angles at vertex 3 are constant, then the following also holds: $$\alpha^2\cdot \mathbb{cotg}^2341 - \beta^2\cdot \mathbb{tanh}^2(l_{24}) = 1$$ where $$\alpha = \mathbb{cotg}431,\quad \beta = \mathbb{cotg}234.$$

For the tetrahedron under the conditions described in Gauss's note, Gauss's formula therefore connects the length of the side $24$ with the angle $341$; literally speaking, it says the length of the side $24$ is an inverse hyperbolic function of a trigonometric function of the angle $341$. In his commentary on Gauss's note, Stackel says Gauss missed a factor of $1/2$ that in his expression for the volume differential.

Therefore, my questions are:

• What is the meaning of Gauss's results? how to derive them?
• How to derive a volume function $\Delta(l_{24})$ for the tetrahedron satisfying Gauss's conditions by the analytic procedure outlined by Gauss?