  The focus of this study is to reveal the reason behind a scale problem
detected around 1849 in the historical version of the International Sunspot
Number Series, i.e. version 1 (Leussu et al, Astronomy and Astrophysics, 559,
A28, 2013; Friedli, Solar Phys.291, 2505, 2016). From 1826 to 1848 Heinrich
Schwabe's observations were considered primary by Rudolf Wolf, and a shift of
primary observer from Schwabe to Wolf in 1849 seems to have led to an
inconsistency in the Sunspot Number series. In this study we benefited from
various datasets, the most important being Schwabe's raw counts from the
Mittheilungen (Prof. Wolf's Journals) that have been digitised at the Royal
Observatory of Belgium between 2017 and 2019. We provide a robust
quantification of the detected problem by using classic algebraic calculations
but also different methods such as a method inspired by Lockwood et al (Journal
of Geophysical Research (Space Physics), 119(7), 5172, 2014), hence assigning a
modern k-factor to Schwabe's observations before 1849. We also assess the
implications of this 1849 inconsistency on the International Sunspot Number
series (Versions 1 and 2) before and after 1849.
