DISCUSSION OF TIDES IN BOSTON HARBOR. 
CAMBRIDGE, MASSACHUSETTS, May 2, 1870. 
DEAR Str: I have the honor to submit the following report of a discussion of the Boston dry- 
dock tide observations. On account of the completeness of the set of observations and the 
importance of the tide-station, as representing a peculiar, and, in relation to the tidal theory, a very 
interesting, type of tides, extending along the whole coast of New England, it has been thought 
advisable to give all the labor to the discussion necessary to obtain the most accurate results, and 
also to give a somewhat full report of them. Some applications of the results have also been made 
to theory, and practical formule and tables constructed for the prediction of the tides. 
THE OBSERVATIONS AND LOCALITY. 
1. The series of observations used in this discussion embrace a period of nineteen years, com- 
mencing with the 1st of July, 1847, and extending to the 1st of July, 1866. The series is very 
nearly complete, the observations of the times and heights of both high and low water having been 
made with great regularity both day and night, and for every day of the week; so that it rarely 
happened that even a single observation was lost. Only on two occasions were there any interrup- 
tions of any consequence in the whole series. During the latter part of May, and the first part of 
June, 1854, one week’s observations were lost, owing to the observer’s being on duty to check the 
mob at the time of the Burns trial. The observations of ten days, also, of the latter part of Jan- 
uary, 1865, were lost on account of the illness of the observer. 
The first part of the series of observations was made with a tide-gauge consisting of marks des- 
ignated by copper figures set in the stone wall at the entrance to the dry-dock. The observer 
commenced usually about a half hour before each high and low water and noted the readings every 
five minutes until the stand, then the duration of the stand, and then again he noted the readings 
every five minutes until he felt sure the tide had changed. In 1860 a box-gauge was substituted, 
which was used until the series was closed, on the 30th of September, 1866, the full series having 
been commenced in June and continued about nineteen years and four months. 
The observations seem to have been accurately made, so far as can be judged from the group- 
ing of them in the discussion; but this is rather an uncertain criterion, since numerous astronomi- 
cal inequalities, and also meteorological irregularities, necessarily cause a considerable range in the 
observations of each group. The most accurate test of the faithfulness of the observer is found in 
the slight irregularities caused by the diurnal tide. These irregularities in both the heights and 
times of the tide at the Boston dry-dock are quite small, and only sensible to ordinary inspection of 
the observations at certain periods; yet these irregularities can generally be readily distinguished 
in the recorded observations, amid all the other numerous irregularities, at such times as theory 
indicates that they should be most easily seen. It is not probable that the observer knew either 
the periods of these irregularities, or their character, and hence their occurring in the record must 
have resulted from a careful observation of the tides. 
This discussion has not been made from the original records of the observer, but from what are 
called the first reductions of the tide observations, in which are given the apparent times of the 
moon’s meridian transit, aud the apparent times and absolute heights of high and low water, and 
the lunitidal intervals, as obtained from the observer’s record. These reductions, so far as I had 
any means of testing them, seem to have been carefully and accurately made. 
2. On account of the shallowness of the greater part of the harbor, and even of the bay for 
many miles beyond, and the interruptions of numerous islands, and the narrowness of the channel 
leading to the Boston dry-dock, the tide-station must be regarded as a somewhat inland one, and 
some of the characteristics of river-stations should be found in the observed tide. It is seen, from 
consulting the Coast Survey chart of the bay and harbor, that the average depth of the channel in 
