Please read the following table in HTML format from a webpage and then answer the question in detail according to the table. Each table cell is defined by a <td> and a </td> tag. Each table row starts with a <tr> and ends with a </tr> tag. 'rowspan=m' and 'colspan=n' represents a cell spaning over 'm' rows and 'n' columns.

Webpage title:
English brewery cask units
Table title:
Chart
Table:
<table id='hittask' contenteditable='False'>
<tr> <th colspan=1 rowspan=1 > gallon </th><th colspan=1 rowspan=1 > firkin </th><th colspan=1 rowspan=1 > kilderkin </th><th colspan=1 rowspan=1 > barrel </th><th colspan=1 rowspan=1 > hogshead </th><th colspan=1 rowspan=1 >  </th><th colspan=1 rowspan=1 > Year designated </th></tr>
<tr> <td colspan=1 rowspan=4 >  </td><td colspan=1 rowspan=3 >  </td><td colspan=1 rowspan=2 >  </td><td colspan=1 rowspan=1 >  </td><td colspan=1 rowspan=1 > 1 </td><td colspan=1 rowspan=1 > hogsheads </td><td colspan=1 rowspan=4 >  </td></tr>
<tr> <td colspan=1 rowspan=1 > 1 </td><td colspan=1 rowspan=1 > ​1 1⁄2 </td><td colspan=1 rowspan=1 > barrels </td></tr>
<tr> <td colspan=1 rowspan=1 > 1 </td><td colspan=1 rowspan=1 > 2 </td><td colspan=1 rowspan=1 > 3 </td><td colspan=1 rowspan=1 > kilderkins </td></tr>
<tr> <td colspan=1 rowspan=1 > 1 </td><td colspan=1 rowspan=1 > 2 </td><td colspan=1 rowspan=1 > 4 </td><td colspan=1 rowspan=1 > 6 </td><td colspan=1 rowspan=1 > firkins </td></tr>
<tr> <td colspan=1 rowspan=1 > 1 </td><td colspan=1 rowspan=1 > 8 </td><td colspan=1 rowspan=1 > 16 </td><td colspan=1 rowspan=1 > 32 </td><td colspan=1 rowspan=1 > 48 </td><td colspan=1 rowspan=2 > ale gallons </td><td colspan=1 rowspan=4 > (1454) </td></tr>
<tr> <td colspan=1 rowspan=1 > = 4.621 L </td><td colspan=1 rowspan=1 > = 36.97 L </td><td colspan=1 rowspan=1 > = 73.94 L </td><td colspan=1 rowspan=1 > = 147.9 L </td><td colspan=1 rowspan=1 > = 221.8 L </td></tr>
<tr> <td colspan=1 rowspan=1 > 1 </td><td colspan=1 rowspan=1 > 9 </td><td colspan=1 rowspan=1 > 18 </td><td colspan=1 rowspan=1 > 36 </td><td colspan=1 rowspan=1 > 54 </td><td colspan=1 rowspan=2 > beer gallons </td></tr>
<tr> <td colspan=1 rowspan=1 > = 4.621 L </td><td colspan=1 rowspan=1 > = 41.59 L </td><td colspan=1 rowspan=1 > = 83.18 L </td><td colspan=1 rowspan=1 > = 166.4 L </td><td colspan=1 rowspan=1 > = 249.5 L </td></tr>
<tr> <td colspan=1 rowspan=1 > 1 </td><td colspan=1 rowspan=1 > ​8 1⁄2 </td><td colspan=1 rowspan=1 > 17 </td><td colspan=1 rowspan=1 > 34 </td><td class="highlighted"  colspan=1 rowspan=1 > 51 </td><td class="highlighted"  colspan=1 rowspan=2 > ale gallons </td><td class="highlighted"  colspan=1 rowspan=2 > 1688 </td></tr>
<tr> <td colspan=1 rowspan=1 > = 4.621 L </td><td colspan=1 rowspan=1 > = 39.28 L </td><td colspan=1 rowspan=1 > = 78.56 L </td><td colspan=1 rowspan=1 > = 157.1 L </td><td class="highlighted"  colspan=1 rowspan=1 > = 235.7 L </td></tr>
<tr> <td colspan=1 rowspan=1 > 1 </td><td colspan=1 rowspan=1 > 9 </td><td colspan=1 rowspan=1 > 18 </td><td colspan=1 rowspan=1 > 36 </td><td colspan=1 rowspan=1 > 54 </td><td colspan=1 rowspan=2 > ale gallons </td><td colspan=1 rowspan=2 > 1803 </td></tr>
<tr> <td colspan=1 rowspan=1 > = 4.621 L </td><td colspan=1 rowspan=1 > = 41.59 L </td><td colspan=1 rowspan=1 > = 83.18 L </td><td colspan=1 rowspan=1 > = 166.4 L </td><td colspan=1 rowspan=1 > = 249.5 L </td></tr>
<tr> <td colspan=1 rowspan=1 > 1 </td><td colspan=1 rowspan=1 > 9 </td><td colspan=1 rowspan=1 > 18 </td><td colspan=1 rowspan=1 > 36 </td><td colspan=1 rowspan=1 > 54 </td><td colspan=1 rowspan=2 > imperial gallons </td><td colspan=1 rowspan=2 > 1824 </td></tr>
<tr> <td colspan=1 rowspan=1 > = 4.546 L </td><td colspan=1 rowspan=1 > = 40.91 L </td><td colspan=1 rowspan=1 > = 81.83 L </td><td colspan=1 rowspan=1 > = 163.7 L </td><td colspan=1 rowspan=1 > = 245.5 L </td></tr>
</table>
Question:
How many ale gallons was hogshead defined as in 1688?
Answer:
In 1688 the ale hogshead was defined to be 51 ale gallons (235.7 l).