  Solar meridian observations have been used to evaluate the solar activity of
the past. Some important examples are the solar meridian observations made at
the Basilica of San Petronio in Bologna by several astronomers and the
observations made by Hevelius published in his book Machina Coelestis. However,
we do not know whether these observations, which were not aimed to estimate the
solar activity, are reliable for evaluating solar activity. In this paper, we
present the marginal notes about sunspots that are included in the manuscripts
of the meridian solar observations made at the Royal Observatory of the Spanish
Navy during the period 1833-1840. We compare these observations with other
solar activity indices such as sunspot area and number. Our conclusion is that
solar meridian observations should be used with extreme caution to evaluate
past solar activity.
