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:
List of Sherlock Holmes episodes
Table title:
The Return of Sherlock Holmes (1986–1988)
Table:
<table id='hittask' contenteditable='False'>
<tr> <th colspan=1 rowspan=1 > No. </th><th colspan=1 rowspan=1 > Title </th><th colspan=1 rowspan=1 > Based on </th><th colspan=1 rowspan=1 > First broadcast </th><th colspan=1 rowspan=1 > Synopsis </th></tr>
<tr> <td colspan=1 rowspan=1 > 14 </td><th colspan=1 rowspan=1 > "The Empty House" </th><td colspan=1 rowspan=1 > "The Adventure of the Empty House" </td><td colspan=1 rowspan=1 > 9 July 1986 </td><td colspan=1 rowspan=1 > Holmes returns to England, to Watson's shocked surprise, and must stop a member of Moriarty's gang who is trying to assassinate him. </td></tr>
<tr> <td colspan=1 rowspan=1 > 15 </td><th colspan=1 rowspan=1 > "The Priory School" </th><td colspan=1 rowspan=1 > "The Adventure of the Priory School" </td><td colspan=1 rowspan=1 > 16 July 1986 </td><td colspan=1 rowspan=1 > A rich duke reluctantly allows Holmes to search for his ten-year-old son who is missing from his boarding school along with the German tutor. </td></tr>
<tr> <td colspan=1 rowspan=1 > 16 </td><th colspan=1 rowspan=1 > "The Second Stain" </th><td colspan=1 rowspan=1 > "The Adventure of the Second Stain" </td><td colspan=1 rowspan=1 > 23 July 1986 </td><td colspan=1 rowspan=1 > The Prime Minister asks Holmes to help recover a stolen letter which could lead to war if released. </td></tr>
<tr> <td colspan=1 rowspan=1 > 17 </td><th colspan=1 rowspan=1 > "The Musgrave Ritual" </th><td colspan=1 rowspan=1 > "The Adventure of the Musgrave Ritual" </td><td colspan=1 rowspan=1 > 30 July 1986 </td><td colspan=1 rowspan=1 > While vacationing at the estate of his old university classmate Sir Reginald Musgrave, Holmes is called in when the scholarly butler disappears after being discovered reading an old document about a mysterious family ritual. </td></tr>
<tr> <td colspan=1 rowspan=1 > 18 </td><th colspan=1 rowspan=1 > "The Abbey Grange" </th><td colspan=1 rowspan=1 > "The Adventure of the Abbey Grange" </td><td colspan=1 rowspan=1 > 6 August 1986 </td><td colspan=1 rowspan=1 > Lord Brackenstall has been murdered and his battered wife says that a thief and his sons were the culprits: wine glasses convince Holmes otherwise. </td></tr>
<tr> <td colspan=1 rowspan=1 > 19 </td><th colspan=1 rowspan=1 > "The Man with the Twisted Lip" </th><td colspan=1 rowspan=1 > "The Man with the Twisted Lip" </td><td colspan=1 rowspan=1 > 13 August 1986 </td><td colspan=1 rowspan=1 > Watson finds Holmes in an opium den, where he has been sent to look for Neville St Clair, who is suspected to have been killed by a beggar. </td></tr>
<tr> <td colspan=1 rowspan=1 > 20 </td><th colspan=1 rowspan=1 > "The Six Napoleons" </th><td colspan=1 rowspan=1 > "The Adventure of the Six Napoleons" </td><td colspan=1 rowspan=1 > 20 August 1986 </td><td colspan=1 rowspan=1 > Holmes is contacted by Inspector Lestrade after the police are baffled by a series of burglaries serving only to steal and smash identical busts of Napoleon. </td></tr>
<tr> <td colspan=1 rowspan=1 > 21 </td><th class="highlighted"  colspan=1 rowspan=1 > "The Sign of Four" </th><td colspan=1 rowspan=1 > The Sign of the Four </td><td class="highlighted"  colspan=1 rowspan=1 > 29 December 1987 </td><td colspan=1 rowspan=1 > Mary Morstan has received a pearl in the post every year since her father's disappearance; this leads Holmes and Watson to the truth about a secret pact between four convicts during the Indian Rebellion of 1857. </td></tr>
<tr> <td colspan=1 rowspan=1 > 22 </td><th colspan=1 rowspan=1 > "The Devil's Foot" </th><td colspan=1 rowspan=1 > "The Adventure of the Devil's Foot" </td><td colspan=1 rowspan=1 > 6 April 1988 </td><td colspan=1 rowspan=1 > In Cornwall, Holmes investigates when a woman mysteriously dies with a horrified look on her face and her two brothers are driven insane. </td></tr>
<tr> <td colspan=1 rowspan=1 > 23 </td><th colspan=1 rowspan=1 > "Silver Blaze" </th><td colspan=1 rowspan=1 > "The Adventure of Silver Blaze" </td><td colspan=1 rowspan=1 > 13 April 1988 </td><td colspan=1 rowspan=1 > Holmes investigates when a prized racehorse goes missing and his trainer is found bludgeoned to death. </td></tr>
<tr> <td colspan=1 rowspan=1 > 24 </td><th colspan=1 rowspan=1 > "Wisteria Lodge" </th><td colspan=1 rowspan=1 > "The Adventure of Wisteria Lodge" </td><td colspan=1 rowspan=1 > 20 April 1988 </td><td colspan=1 rowspan=1 > An amateur cartographer who stayed with another map enthusiast seeks help when his host vanishes in the middle of the night. </td></tr>
<tr> <td colspan=1 rowspan=1 > 25 </td><th colspan=1 rowspan=1 > "The Bruce-Partington Plans" </th><td colspan=1 rowspan=1 > "The Adventure of the Bruce-Partington Plans" </td><td colspan=1 rowspan=1 > 27 April 1988 </td><td colspan=1 rowspan=1 > Holmes helps Mycroft when stolen top secret blueprints for a submarine are found on the body of a civil servant found dead on the tracks of the Underground. </td></tr>
<tr> <td colspan=1 rowspan=1 > 26 </td><th class="highlighted"  colspan=1 rowspan=1 > "The Hound of the Baskervilles" </th><td colspan=1 rowspan=1 > The Hound of the Baskervilles </td><td class="highlighted"  colspan=1 rowspan=1 > 31 August 1988 </td><td colspan=1 rowspan=1 > Sir Charles Baskerville was found dead with a horrible expression on his face after seeing a mysterious demonic spirit of a gigantic hound that has haunted his family. </td></tr>
</table>
Question:
What dates did "The Sign of Four" and "The Hound of the Baskervilles" aired, respectively?
Answer:
"The Sign of Four" and "The Hound of the Baskervilles", aired on 29 December 1987 and 31 August 1988 respectively.