ICOADS Web information page (Tuesday, 07-Jan-2014 23:59:33 UTC):

Figure 3b

Figure 3b. Numbers of observations (upper panel) and 2° boxes (lower panel) of AT per month (1820-1859) used for the enhanced statistics (otherwise as for Fig. 1a).

[Links to detailed][Report-rejection Problem]

U.S. National Oceanic and Atmospheric Administration hosts the icoads website privacy disclaimer
 Document maintained by icoads@noaa.gov
Updated: Jan 7, 2014 23:59:33 UTC
http://www.icoads.noaa.gov/zq_fig3b.html