This is a very interesting manuscript that describes the Challenger and Gazelle expeditions that were carried out in 1869-1871 during a time of remarkable evolution of science, particularly oceanographic sciences. Until recently the Gazelle expedition was largely ignored and is important that the two expeditions be described and compared.

The descriptions of the vessels, the naval and scientific parties and the subsequent data analyses and reports explained the differences in the scientific impact of the two expeditions. However, it would be good if the section on how the sailing instructions were formulated was expanded.  It was also difficult to determine exactly what measurements and how many of them were obtained from each expedition.

Until recently naval hydrographic services, were typically charged with providing bathymetry, tidal amplitudes, surface current charts and meteorologic data that are used for surface navigation. Preparation of these charts for navigation did not require knowledge of the distribution of temperature below the surface. “Naval Surveyors” were presumably responsible for getting the data for these products. Making these observations for the hydrographic services would have been the responsibilities of Nares, Tizard and von Schleinitz during their earlier naval careers. The advent of submarine cables also would require knowledge about ocean depths and the composition of the bottom, but not sub-surface temperature data. Maury’s Brussels 1853 conference laid out a uniform procedure for reporting only meteorological data. That is, it did not set up a procedure to report sub-surface oceanographic data or provide a rational for these observations. It appears that preparation of these navigation charts and “showing the flag” were the justification for the Gazelle. It is clear from the first sentence of the Thompson and Murray cruise report that sub-surface measurements were a primary objective of the Challenger expedition. The present manuscript discusses conversations between Thompson and Carpenter about making biological measurements at the sea floor, which explains the biological sampling. The Challenger subsurface temperature measurements were revolutionary. Was the focus on deep ocean measurements due to George Richards who was the head of the naval hydrographic service and also a member of the Royal Society? Maury (1855) did describe the surface currents and temperatures of the North Atlantic and noted the differences in temperature (and density) along the path of the Gulf Stream and attempted to explain how the currents were generated. Was the scientific rationale for these sub-surface temperature measurements purely scientific exploration or was there some underlying basis to make these observations? The present manuscript mentions that a Royal Society committee prepared a report that was used to justify and set up the Challenger cruise, but there is no entry in the reference list to this report. Does this report set out the justification for the sub-surface measurements? Someone deserves the credit for proposing these observations and it would be good to make this clear in the present manuscript.

It is difficult to discern what and how many measurements were made from each expedition. The equipment for each ship is given, but how they were used in not clear.  For example, both ships had hemp rope for sub-surface measurements. I expect that both ships used the rope for bottom depth measurements, and the Challenger used it to measure sub surface temperature. I did look through the references to find out how the temperature measurements were made. It might be interesting to discuss this in this manuscript. Did the Gazelle make other sub-surface measurements? Specific volume measurements were made on both expeditions. I was unable to find in the references how the specific volume measurements were made?  It appears from the Gould and Cunningham (2021) reference that these measurements were only made from surface bucket samples which is not surprising since the capability to collect sub-surface samples had not yet been invented. From the Roemmich, Gould and Gilson (2012) paper one can get an estimate of the number of sub-surface casts.  Similarly, one can get an estimate of the number of surface bucket measurements from the Gould and Cunningham (2021) paper. It would be useful to have a table that gave the list of physical measurements for each expedition and the number of these measurements. Similarly, it would be good to list the types of biological measurements that were made, but this might get complicated if one includes the terrestrial samples. Were there many deep sea benthic biological samples that addressed the suggestions from Thompson and Carpenter? That is, it would be beneficial to have a summary list of the sampling from each cruise.

If these two issues are addressed, I strongly recommend that this manuscript be published.

Challenger/Gazelle review

1. In the mid 1880s two very similar European naval vessels undertook multi-year circumnavigations with a main objective of making what we would now call oceanographic observations.  One of them, the British HMS Challenger, is still remembered and celebrated throughout the oceanographic community as making a very significant contribution to the development of ocean science. In contrast, the German SMS Gazelle is almost forgotten even in its native Germany. As we approach the 150^thanniversary of both cruises, John Gould attempts to explain why the two ventures have left such different records and, at the same time, tries to bring the Gazelle results to the notice of a larger audience than has hitherto been the case. In my opinion he succeeds in both objectives and also puts the two cruises into a late nineteenth century political context which, to my knowledge, has not been done so explicitly before. Therefore, with the exception of some relatively minor, mostly typo changes (see section 3 below), I strongly recommend publication.
2.  Another reviewer has made some very valid points about possible additions to the current ms, particularly with regard to more information about how the sailing instructions were developed, why the Challenger took so many sub-surface temperature measurements and further details of the number and distribution of the various samples and data collected by the two voyages. I disagree; in my opinion the paper is already long enough, with quite a lot of biographical information that has already been published elsewhere. Despite this, I support its inclusion because it will make much more interesting and informative reading for someone with limited pre-knowledge of early oceanography. However, I feel the addition of further details such as those mentioned above would be superfluous for the following reasons. First, the background to the Challenger voyage was extensively covered in Wyville Thomson’s introduction to the reports and widely quoted subsequently (and are available on line, of course).  In the case of the Gazelle’s instructions I feel that the author has made a decent fist of explaining where the orders came from on the basis of what seems to be relatively sparse documentation. Second, subsurface temperature measurements, albeit relatively few and often not very reliable, had been taken by many exploration cruises certainly since the early 1800s so any major new cruise would  certainly have been expected to continue the practice. Furthermore, however, they became particularly desirable after the discovery of widely varying near-bottom temperatures during the Lightning cruise of 1868 and the subsequent Porcupine cruises, and the bearing they had on the growing controversy about the causes of deep sea circulation, well publicised by Thomson in The Depths of the Seapublished as the Challengersailed. Finally, while a little more detail of the Gazelle data gathering regime might be useful, to do the same for the Challenger would be a huge undertaking, would add very significantly to the paper’s length and, in any case, is available elsewhere.

1. The following is a list of minor, mostly typographical, changes I suggest.

85 The origin of species (not origin of the species)

474 “Holderness, a position…

560 compare spelling line 507

677 “…isopods and other crustaceans…”

956   “….in the ocean. The leading innovator….”

972 “…..other experimental devices. One, designed by Siemens…”

1. “Arguably, had the Challenger….
2. “Thus, the major focus….”
3. “...were not safe…”
4. “...seawater, thus freeing…”
5. “...called on…” not “called in”
6. Should have a note here that, after the material had been studied and reported on, most of the marine material was deposited at the Natural History Museum in London, where it still is.

Tony Rice

Alton, Hants,  July, 2022