In contrast with the previous year, the winter of 1647–48 was one of the mildest that Descartes had experienced since his arrival in the United Provinces. In other circumstances, this would have been a welcome change for the resident Frenchman. However, in January 1648 Descartes was recording variations in barometric pressure and comparing his results to those obtained by Mersenne in Paris. For scientific reasons, therefore, he would have welcomed a cold spell. His correspondence with Mersenne in the early months of 1648 showed a continued interest in the issues that had been raised by Pascal during the previous summer, and in the debate about barometric measurements and the so-called vacuum at the top of a Torricelli tube. He even claimed to have been looking forward to seeing a new book on the subject by Father Noël, entitled Gravity Compared, despite his legendary reluctance to read other people's publications.

What was even more evident, at the beginning of 1648, was Descartes' indecision about where he planned to live during the following years. He had promised Picot to spend the winter of 1648–49 with him in Paris. This plan was now in the process of being changed.